I was told to discuss my use of two contested UUs here for argument's sake. My current UU team uses Leafeon and Pinsir, what I have found is as follows:
Leafeon has amazing stats but its movepool limits it so much that it isn't funny. Its best type coverage is Grass+Normal which is totally walled by anything steel type. It's a bit like Tangrowth on the special side, any decent STAB ice or fire special attack is pretty much sure death. I don't keep it in on water types half of the time because they carry ice moves. I kept it in verses a vaporeon aurora beam (yes, he was a noob) and it did about 72% HP damage. Leafeon is a very reliable counter to Quagsire and Gastrodon, but is generally best used as a bulky and speedy BPer. Scyther also destroys it if he's allowed in UU, it destroys pretty much anything it wants after a SD. Leafeon's abysmal coverage and only decent speed ensures that even a fully set up leafeon is fairly easily countered compared to even other UU setups. I have to say though, if his movepool didn't consist of two decent physical moves with 0 synergy this would be OU material. By the way, most Leafeons do not carry return so anything with a grass resist generally works as a sound defense. The main reason to use leafeon over ninjask is that while it does not pass speed, it will survive to baton pass multiple times rather being a one trick worthless pony like Ninjask.
Good counters: Pinsir, Scyther, Aggron (provided it hasn't set up too much), Torkoal, Magnetric (provided it is not set up it's easy to switch in on SD/Leaf Blade), anything with roar to stop the baton pass buggery.
Pinsir is like heracross lite, problem is he is x2 SR weak (which is pretty bad in UU), his bug type attack is 25% weaker than Megahorn and his close combat is not STABed. Scarfed versions are easily countered by Torkoal with his fire attacks and burns (remember, no guts ability which ruins his status absorbing potential), Aggron, anything bulky with somthing that will hit SE will suffice. Banded versions can be pretty easily outsped and killed by anything with a flying, rock, or fire attack (common types in UU).
Good counters: Torkoal, Aggron, any status user that can take a hit (rotom works well if it can swtich in on a scissor), Scyther and Magnetric destroy the slower CB versions although maggie has problems switching in.