I come down on the "It's a legitimate D&D mechanic" side of this debate. Yes, it's powerful in the right context. Pushing someone into a bottomless pit or into lava immediately eliminates them. It ALSO prevents looting (or, in the specific case raised, decapitation). I've loaded a game after accidentally knocking a mimic off a ledge with an Eldritch Blast after it stole an important weapon.

(You want OP? Try starting a fight against a certain goblin leader by lining up half the people in the room with a Thunder Wave to knock them all into a spider pit.)

But the reason I come down on the 'legitimate mechanic' side is because I've seen it happen in pen-and-paper sessions. A group of low-level adventurers traipsing around the Underdark stumbles across a creature that - from appearances - we presume to be evil and VERY powerful. (We didn't finish that campaign, but my inference is that 'evil' was wrong...and that it was indeed a very powerful creature.) My character wanted to try talking to it, but got outvoted by the bloodthirsty neanderthals in the party, who charged in, attempted to push it off the cliff on the first turn of combat, and rolled a natural 20, sending this incredibly powerful (and plot-important) NPC off the edge into the depths of the Underdark.

Arguably, enemies should try to be a bit more tactical about avoiding being positioned to get pushed off a cliff. And I won't weigh in on the distance of the push. But in principle, the mechanic is fine.