Maybe I just have weak legs
Seriously, you're right...any lock done to fast will mess up the joint (part of the reason I stopped going to Aikido classes was because my wrists were killing me...all the time).
There are a few leg locks, like the heel hook, that are EXTREMELY sudden. It's one of those bizaare locks where if you wait to feel the pain to tap, somethings already been busted.
For whatever reason, I hear more stories about people getting hurt with leg locks than not...maybe it's just me.
They are a phenomenal tool, but like anything else, if you train dumb, you'll probably get hurt.
Tell the Spartans, stranger passing by,
that here obident to their laws we lie
- Inscription at the site of Thermopylae