Im gonna talk about MSwampert rn, and I must say I overhyped him to shit.
I think MSwamperts biggest flaw is his Mega transformation turn. He doesn't get his stat boosts, his ability doesn't hit in, and he needs to have Rain Dance or a viable Rain setter (Politoed, whatever). Speaking of support, this thing needs a decent amount. Playing with MSwampert, it's not as powerful as people say it is. It has dual stabs, yes. They do a decent amount of damage, but his other moves (Some being much more important than others) like Ice Punch don't do a lot of damage. Its probably because most Megas that MSwampert likes to take out (bar MSceptile and MBeedrill) does not get KO'ed by it.
Though, it is decent, it requires too much support for me (Rain setter, optional Rain sweeper, Ferrothorn) and its 150 attack actually isn't that impressive, as most megas that hit hard have things like Aerialate/Pixilate (increases their Normal moves by 1.3x and STAB) Huge Power, etc.
Im kind of disappointed, but thats my opinion on MSwampert. Its still decent, but it was way overhyped.