Monks in D&D are no joke. I've gotten torn up by them before. That class has actually been around since the very beginning. I think I might replay as one.
Interesting, the general consensus in rpg spheres I hang out in is that monks are cool and flavorful but dead last mechanically.
I've never run the numbers myself, or even played a monk, but I can say at higher levels "stunning strike" beginners pretty impossible to pull off with how high enemies con saves all end up being.
The picture I've been getting is that they are considered pretty awful by most 5E experts, but a combination of rules changes, class tweaks and magic items make them much stronger in BG3.
They’re bad unless played a very specific way with some specific gear to help them dig into that niche they fit in. They don’t just do great no matter what like a lot of other classes
"Mystic", to be fair, was what the OG system called them, but then AD&D was the first edition to use the term "monk", and each subsequent edition has had some form of the class present in either the PHB or supplemental texts. Technically, 1st Edition D&D did not have Monk as a player class.