Before my group understood the rule about not stacking temporary HP, my lvl 10ish warlock was in an area that was attacked by swarms of insects. But tons of insects - like no empty spaces in a square mile going 100 feet up. So I cast a fireball into the mass of bugs and, I don't remember the exact calculation we used to figure out how many swarms I killed with it, but Dark One's Blessing gave me several hundred temp HP. For one action and a spell slot.
Obviously that particular scenario isn't likely to come up in this game, but any combat where you can one-shot enemies with Eldritch Blast, for example, is going to get out of hand quickly. That's not counting any other potential sources of temp HP. There's no question that stacking temp HP can be overpowered.
