The ZF s5-42 (which is 99% of what you'll find available) isn't a bad transmission, per se. It's rated for 420ft-lbs of input torque, but it'll take a /lot/ more than that.
Justin @ R&D had one behind his 400+RWHP IDI(something like 1,000 ft-lbs of torque), and the transmission didn't blow up. Clutch had a real hard time holding that power, though.
I'm currently running 460 at the wheels, which is a bit more than that at the crank. No issues on my transmission.
The main downside to the ZF transmission are that the synchros are 'weak'. I'm not sure if they are weak in general, or simply the rest of the transmission outlived it's design life by a long time, but most any ZF transmission you find will have some worn out synchros.
Common symptoms are:
1. Hard to get into second gear when moving. I've got a couple which will do it if you rev-match, but if you try to force it, they'll grind instead of going into gear.
2. 3rd gear being 'notchy'(hearing a few teeth bouncing off before it goes in). Also related to worn synchros.
3. Reverse synchro being shot - this results in it being hard to put into reverse(also possible to have it pop out of reverse, but I've not seen that).
What I see is where you put the transmission into reverse, hear it not quite go in, let the clutch out just a hair while holding the gearshift and hear a 'clunk' as it finally engages.
That, or having to put it back in neutral, let the clutch out for a second, press back in and then quickly put it into reverse(so the input shaft was still spinning) at which point it goes in just fine.
Also, I've got a couple of transmissions with bearing noises from something or other; rest of the transmission seems just fine though.
Despite all that, they tend to work just fine, handle high torque, and I'm quite happy with mine, even with the synchro-issues.
Someday I'll try to rebuild one and fix it, but it's not something that's in the 'must be fixed' category.