Mega Feraligatr is a ferocious sweeper, especially when used under rain. To give players an idea of just how powerful this Pokémon is:
+2 252+ Atk Tough Claws Feraligatr Waterfall vs. 252 HP / 252+ Def Slowbro in Rain: 219-258 (55.5 - 65.4%) -- guaranteed 2HKO after Leftovers recovery
It has the ability to 2HKO one of its best counters after one Swords Dance under the rain. It is almost impossible to revenge kill considering its Tough Claws boosted Aqua Jet which has priority over other moves. It can also function well as a Dragon Dance sweeper but it already gained a speed boost with the Mega Evolution so, in general, the Swords Dance set is better.
Although Mega Feraligtr is a beast, there are counters to it. Jellicent is immune to Water-type attacks with its ability Water Absorb, is immune to Superpower thanks to its Ghost typing, and doesn't really mind taking an Ice Punch. It can respond by crippling it with a Will-O-Wisp, while only being afraid if Mega Feraligatr has Crunch. Chesnaught can counter variants without Ice Punch. Vaporeon is also immune to the Water-type attacks and doesn't take much damage from any of its coverage moves either.