I've been testing this for a while now and Megaman X1-3, MM7, and Rockman & Forte all work really well in my experience, having done full playthroughs of each. The only major bug is in X1, which requires the "Max" option rather than "Compatible", but things like that are to be expected of the one-size-fits-all/minimalist options Libretro cores tend to have.
If using Compatible, Boomer Kuwanger's stage will have corrupt BG graphics when re-entering it after beating it. (Armored Armadillo or Storm Eagle can have this issue too, if you're using older builds with different overclock values.) The only other bug I've found at all is at the very end of X2's intro sequence, the wireframe "X" animates too quickly, prematurely cutting off the music. X3 is the same way, but it doesn't cut off the music. (Just lucky timing I think, I would be surprised if they actually tied screen fade to the music ending.)