Terrible.

BII and SheepShaver don't work with macsbug, the system crashes when triggering a macsbug breakpoint.

MESS can run macsbug, but there's other issues. 1) there's no power button or NMI interface in MESS to break into the debugger. 2) keyboard and mouse inputs in MESS are a bit flakey and unreliable. It makes trying to code/debug/really use the emulator frustrating at best.