We don't, yeah, except from the reactions to stuff so far.
If you look at the reddit, it's mostly younger players a lot of whom started with 5E (some 3E or 4E, but usually when they were fairly young with the 3E ones), mostly 20s-30s, and they had extremely positive reactions to all the race stuff, like all of it, in Tashas, and in MoM. That all made complete sense to them and you can see in discussions of MoM it's just taken for granted that that's fine.
But the monsters in MoM? They're mad about them - not because they're "not evil" or something, they don't mind that sort of thing - no they're made because the the monsters are:
A) Being "simplified"
and
B) Being "nerfed"
And that's what their annoyance centers around - stuff being "dumbed down". Which like, to an RPG vet like us, that's not what's happening, we can see that there's a specific goal, this isn't mindless dumbing down, and I suspect we're probably less prone to being annoyed by stuff like monsters having fewer spells.
But these peeps? They mad! Be interesting to see how they'll react to class changes, if they happen. They were very pro-Tashas, generally, so there's that bit of evidence, but major changes may be more or less popular.