Any advice for a Hexblade warlock

I am the DM. I allow UA in my campaigns because it helps to see them played. I have player who is about to play a hexblade warlock. Looking at the material it seems like there are 30 invocation (I am sure I am exaggerating here) to pick from with the extended ones and it seems they can do a lot. I am still going to allow it as I don't like when people declare something broken before seeing it in action but if you have had one in your games or played one is there anything I should be on the look out for so my encounters are still fun and dangerous? Or things that the first time it is used I will go wtf? but it is actually ok?