All good questions - and every person has different needs.
The fact is each potential adopter either needs to go through and comb over the compatibility page (not what I would expect with a drop-in replacement); or just try and see if it works, and be surprised later if they run into an unimplemented module because they were operating under an incorrect assumption.
The question isn't just how many? but also which ones?. Anecdotally, I've been test driving bun for a large application with millions of existing customers. So far, the only issue I've had isn't with bun per se, it's with patch-package not yet supporting the lock file (bun.lockb).
Bun is without question faster than yarn by a wide margin. And say this as someone who really loves yarn.