Lol at UMP and Scar-H being overpowered. UMP could be considered overpowered IF you are playing HARDCORE, otherwise it is fine. Sure, it is better than the Uzi, but the P90 and Vector are also viable weapons.
There are definitely weapons that are superior to others stat wise (ACR > M4), but you get plenty of options in MW2. There are many viable weapons, so everyone can have their fun (unless you really like the F2000).