I think the hard part to the clinch is keeping it locked on so that you can keep blasting the victim with knees until he's out cold. To do that well & consistently you need to lock the clinch on firmly and keep swinging the guy around to keep him off-balance so he can't go for a takedown or counter the clinch. Best demonstration of that was Anderson Silva ragdolling Franklin around the cage while blasting him with knees.
Locking the clinch on isn't enough, that'll give you the chance to land maybe 4-6 knees before the other guy finds a counter to break the clinch. You need to swing the guy around & keep him off-balance if you want to hit him with 20 knees in Anderson Silva style or do what Wandy did to Rampage in Pride. And it's the swinging around part that very few fighters have figured out so far, lots of fighters have used the clinch and a number of them have gotten to the part where they lock it on, but they haven't figured out the ragdolling part quite yet.
Shogun for instance has used the clinch in a bunch of his fights and done knee & leg kick combos from it, but he doesn't swing guys around by the neck so he only gets 5-6 hit combos out of them. JDS also got a decent clinch on
Roy Nelson and got about 3-4 knees in, could've had a bunch more knees if he pinched down and swung him a bit.