as was also mentioned when talking about Fidelio, Beethoven writes for voices as if he were writing for instruments. This is possible and can even lead to masterpieces, but it is much easier to sing Verdi's Requiem than to sing Beethoven's Ninth... Verdi's Requiem is not "easy," it is easier to sing, meaning it is a piece well-written for voices.
But this discussion can also be made by disconnecting it from the discussion on registers...
These days I am singing a lot of ancient polyphony. And, I don't know if you know, but in ancient polyphony our concepts of registers (basses, baritones, tenors, altos etc...) are largely lost. Ancient clefs do not indicate vocal "types" but rather ranges. Even more so, it was very common for choirs to be made up only of men, which is why we should not consider the alto part as a part for women but as a part for high tenors-contraltinos or for countertenors or for boy soprano altos, and the same goes for sopranos. All our discussions about the passaggi, about the physiology of the voice linked to dynamics would be senseless in a historically informed performance practice. And therefore, compositional stylings do not justify us (that is, composers of the time did not write knowing the things we know today; they reasoned differently).
Let us also add that dynamic markings were not written...
Despite this, ancient polyphony "sounds" impressively. And it is always comfortable, even when it is written or transcribed in a strange way. And all this because it uses harmony and the motion of the parts perfectly. Dissonances are always sustained by long notes, which naturally function as dynamic reinforcements; complex melodic phrases are also sustained by well-defined harmonies; consonances last a long time, giving a sense of release, of relief; phrasing follows the phrase of the text, not just the musical one, and so we have important harmonic moments not only on the tonic accents of the word but also on a main tonic accent of the entire phrase.
Well, this is something I would feel like carving in marble: from the ancient polyphonists, one learns that THE COMPOSER writes the phrasing. It is not an invention or a discovery of the performer. And a well-written piece does not allow the performer to make mistakes.