Sneak Attack:
...doesn't work on elementals. They are swirling energy beings. They might have a "humanoid" shape, but they are NOT humanoid. The compsition of thier bodies is the element they are created from/a part of. If you slice a wave or a landslide or a breeze or a flame in half, does it care? Is it weakened? Does your blow do more damage than that guy with a sword's blow? No. It doesn't. Rationalize all you want. Take physics into account if you want (I guess you could for earth or water, but that's about it). In D&D terms, it doesn't. Is the weapon magic (a spell or magic weapon/item)?! Well, they are creatures brought to and/or held in this world by magic...so, sure, the MAGIC does damage. Not the blow/weapon.
...doesn't work on undead. The corporeal ones are "dead" already...they're so dead, they're UNdead! Cut their spine? So what! Slit their throat? So what! They're not breathing. Hit that spot uder the arm? So what! They don't "feel" pain. They might (for the very kind DM) lose use of that arm...but they're not taking "extra damage." For the incorporeal ones, seems a no-brainer. You can't touch 'em anyway. Can magic (spell or wepaon/item) hit them (the incorporeal and the physical ones)? Yes. Do they do damage? The magic does. Not the blow/weapon. No bonus here.
...on that note, doesn't work on shadow- or light-based beings. Like elementals and/or incorporeal undead, these beings do not have physical form that is effected by anything but magic.
...doesn't work for "extraplanar beings" [demons, devils, angels, slaadi, non-elemental things, etc...). Chalk it up to "supernatural sense" or "divine/abyssal/infernal" power/skill/senses/"magical immunity or resistance", doesn't matter...you just can't "sneak up" on these guys. The force of your blow/weapon is always shunted off to the side, though the magic (of a spell or magic weapon/item) will still "hit" them.
...doesn't work against oozes, slimes or jellies. See elementals. If you slice through jelly does it "hurt" it? Or, in the case of "puddings", keep it from coming back together just as it was?
...doesn't work against "worms" or other segmented creatures (lemures, remorhaz...even, maybe, slugs or snails). They have no neck for a slice, no spine to sever, no joints to separate, just...no. Doesn't work. Slicing it "here" is no different or harmful than slicing it "there."
Annnnnnd...there. I have explained "Sneak Attack" exceptions in the DMG in less than a column. The thief/rogue PCs? They learn what works and what doesn't as they go...as the PCs should in a D&D game.
That doesn't seem unreasonable. None of it does. If you are up against that kind of foe, use your BRAIN! Use you IMAGINATION! Your rogue has a bevy of other skills, aside from "Sneak Attack", that can be of service...Does that make them "as effective in battle"?! NO, it doesn't. Flat out. It doesn't. It's a ROGUE! NOT a fighter! Against orcs and goblins (and elves and humans), or ogres, giants, dragons, etc etc etc...they can do their thing. Or, if nothing else, protect the spellcasters!!! That is certainly NOT a "waste" of actions nor should feel like they are not contributing!