Leglocks aren't necessarily inherently dangerous, but they do come with an added set of risks... Seems to me that it's about awareness of what your opponent is doing, and also how aggressively he/she is applying the technique. The injuries that I've seen in competition from leglocks are almost 100% from one guy not knowing what the other is doing, and then that guy going full blast on the sub.
I think the pain associated with the injuries doesn't always match the repercussion of said injuries, either, although I have had times where I was screaming like a madman after blowing out my knee... I got my knee smashed in a judo tournament about a month ago, and the pain was nauseating.