While I can't speak to the musicological or semiological aspects of this question, the classic Solesmes method seems very practical and the most of the scholas I know try to use it. Without reaching the finesse called for, we strive make a creditable (i.e, musical and prayerful) presentation of the Mass chants, always trying to improve.

There was 1991 article by Peter Jeffery about the 'new Solesmes' method described in the Liber Hymnarius, available at: http://nrvconcert.org/new_chantbooks.pdf

The concluding summary:
"Thus the new Liber Hymnarius and Psalterium Monasticum, despite their innovative attempts to represent the medieval notation more faithfully, are nevertheless the beginning of what is still intended to be a performing edition rather than a truly critical edition. However, this performing edition is of considerable interest to musicologists for a number of reasons" [snip--the author gives 3 reasons]

"Whether used for performances in church, concert hall, or classroom,
this performing edition requires much more than most from the performers who use it. One could perhaps have learned the old Solesmes method merely by reading the preface to the Liber Usualis and listening to Solesmes' classic recordings-at any rate some people tried. But any serious attempt to perform from the new notation will require lengthy study of Cardine's writings, and probably at least some familiarity with the medieval manuscripts themselves. An edition that promotes greater interest in the primary sources should surely be welcomed."

Some time ago, I bought a CD called Chant Wars that was supposed to pit old Roman against Gregorian against I don't recall. I wanted to sit and listen etc. but somehow never did it. (Now I wonder where that CD is!)

I think it was about 18 months ago that I sent out a pdf of a book on the rhythm controversy, something scanned by J Ostrowski. If anyone has that information, it would be good to post.

I have a hard time understanding why this has to be either-or. Cardine's work helps us understand the markings from old manuscripts, which give insight into how scholas might have been conducted (the markings indicate hand movements on the part of the leader), and he found that these markings indicated stresses, elongations, accelerations, and other rhythmic or tempo-related changes that helped the chant reflect the meaning of the text. Does this mean anyone who sings without awareness of these things is wrong? No...and I would think that it would be very difficult to try to make a parish schola chant according to the ancient markings, as you have to agree on each of them and remember to do them, let alone have all singers learn to read them. But is Cardine a somewhat evil force seeking to destroy chant in the Church? Also no. It may be a habit of our times that has us in an A versus B mode on this. The key message from Cardine is, to me anyway, that the text is supreme, and those chanting need to chant in a way that conveys the text, through phrasing and proper syllabic stress, even if the notes are rendered at fairly even lengths.

Here are two samples of "semiological" approach to graduale "Christus factus est":
1. Schola gregoriana pragensis (director David Eben - CD Adoratio crucis)
2. Nova Schola Gregoriana (director Alberto Turco - CD In passione et morte DNJC)
Jeffrey Tucker once pointed me to samples on FSSP site (if you click on link under CD “In coena Domini”, this same graduale will start to play sung by Schola of Our Lady of Guadalupe Seminary). According to Jeffrey’s words this is example of Mocquereau’s rhythm system (or Old Solesmes method, as he calls it) approach.
I personally can’t find connection between the Singing of Schola of Our Lady of Guadalupe Seminary and Mocquereau’s Method.
I don’t think that it is hard to sing after Cardine. If you lead Schola, it is commendable to study his “Gregorian Semiology”, but if you only want to join such a schola, or just sing, you only need to understand few signs and that’s it.
In any case it is best to “meet the chant” personally – to sing together with someone with experience.

I think part of the problem with this whole "controversy" is that it sounds like it's bigger than it really is.

The "semiological approach" (not the best term) does not result in radically different chant. It is not "revolutionary." You don't have to throw out everything you know in order to embrace its suggestions. It is chiefly a refinement of some of the melodies, based on two important chant manuscripts.

The semiological approach does sounds radical and significant if you compare it to the worst practitioners of the "classic Solesmes" approach, i.e. those who sing with a kind of featureless, plodding quality. The Monty Python version of Gregorian chant. The parody that imagines a moldy medieval self-flagellating chant.

Chant was never meant to be like that. Read Gajard's book and Mocquereau's writings. They are quite subtle about rhythm and text, and they insist on a serene vitality.

Moreover, it will be very easy to parody or caricature "semiological" chant as unpredictable, garish, and overly Romantic. And that, too, will have nothing to do with actual "Cardine-style" chant.

So I agree with Scott K. This is not an either-or, and it is a disservice to both approaches to suggest that there is more difference here than actually exists.

I will give the semiologists credit: they have reinvigorated chant and made it exciting. As their predecessors did before them.

The various Solesmes methods may put more emphasis on rythmic flow, on the one hand, or lyric expression, on the other. I'm a complete novice with no right to any opinion of my own but it seems important to, in either case, keep the chant founded upon and proceding from eucharistic adoration.

btw, I'm not drawn to the recordings because the rhythm sounds random. It destabilizes me but I grant that this is more due to my expectations than anything.

The idea of a "French linguistic prejudice" has turned out to be a current topic!
https://www.ccwatershed.org/2022/12/08/gregorian-rhythm-wars-the-limits-of-history/

To say that Mocquereau didn’t understand the Latin accent because he was a native French speaker obviously not a scholarly refutation. To begin with, it seems to me that the French language has a rather strong weighted accent! (Charles Weaver)

"To say that Mocquereau didn’t understand the Latin accent because he was a native French speaker obviously not a scholarly refutation. To begin with, it seems to me that the French language has a rather strong weighted accent! (Charles Weaver)"

In a site of language school:
"in French you don’t stress individual words to emphasize them. Rather, word stress follows a particular pattern, or rhythm, with the last syllable of words at the end of rhythmic groups (or regular phrases) being stressed."

This sounds very similar to the aproach of "mora vocis" at the ends of phrases. If french has a strong weighted accent, it's one that is in the last syllable of incises or phrases and so it's similar to the weight (made of extented duration) of the mora vocis of the various solesmes methods.

in another site the analogy is even more closer:

"The French tonic accent is placed at the end of a group of words. In fact, the French tonic accent is a group accent, not a word accent. In French, there is no variation in the place of the stress nor in their numbers.

But how to determine a group of words in French?
When you speak, you are delivering a message. A message is made up of groups of breath which constitute a complete unity of meaning: a sentence. A breath group can contain several rhythm groups. The voice rises at the end of each rhythm group: this is the intonation. These rhythmic groups constitute partial units of meaning.

The tonic emphasis is always placed on the last syllable of the rhythm group. This accentuation results in an elongation of the syllable."

The french accent is this: elongation of the last syllable of the last word inside a rhythm group. This is exactly how mora vocis is interpreted in the various schools of solesmes. The only exceptions to this is the elongations of episemas in Mocquereu method and the notes that Cardine names "fountain-notes" or the notes he names "pivot-notes" in his book about gregorian chant.

But as a portuguese native speaker and no a french speaker, I elongate the tonic of the latin as I elongate the tonic of portuguese, so I would do for example in the dialogue: "Dóooo-mi-nuus | vo-o-biiiiii-iscuuum. | Et cum Spíiiiii-ri-to tuuuu-o-ooo" (the "dó", of dóminus becomes a fountain-note and "bís" of vobíscum - and "pí" of spiritu and "tu" of tuo - becomes a pivot-note); instead of "dominus vo-obi-is-cuuuuuum. Et cum spirito tu-o-oooooooo" or "dominuuuuus vo-o-bi-is-cuuuum. Et cum spirituuuuu tuo-oooooo". In melismatic chants I would do a long tonic at least two times longer (maybe three ou four times longer) than the next note if the next note is one of many in an unstressed syllable, treating all isolated note of tonic syllables as fountain or pivot that prepares the melismatic development of the unstressed syllable.