Leg locks are very risky because most of the time they do not work in MMA fights, yes I know you highlight Palhares but he is only 1 guy, there are lots of fighters that go for leg locks in UFC and they never get them, they end up losing the position or getting hit a lot.
One of the reasons most guys are not good with leg locks in MMA is because a lot of MMA gyms will not allow leg locks, because they can seriously injure someone and put you on the shelf for ages. I been to 2 MMA established gyms before and they both had leg locks banned, they don't allow it because it can pop your knee easily even if you don't mean to hurt someone and its only training.