They s*ck. The pumper Mikuni butterfly carbs on my Chaparral were flat-out awful. They were a pain to adjust, then stayed in proper tune for only a short time. They also used a LOT of gas. They are better than Tillotsons or Walbros, but that's like saying eating horse manure is better than eating cat manure. There are various types of crap, but it's still crap!
Floater butterfly Keihins are quite simple and sturdy, but are also a little finicky to tune. VERY difficult to find parts for, too.
If you can find a comparable slide-valve Mikuni, that's probably the way to go. In my experience, they remain in tune well, work great, use little gas, and are nearly indestructible.