On the other hand, why shouldn't someone be able to sneak attack a skeleton? His anatomy is like totally obvious
I don't think the "encounter" would be more fun if the DM just said "no you totally get him by surprise but there is no vital spot there so you didn't even scratch him".
Also it wouldn't be fun if ppl had to roleplay how they use the healing kit for a specific injury they have no idea how to treat out-of-game or fail.
Or if they had to pronounce a long complicated phrase in order to cast a spell with verbal components or fail.
Or if they had to remember a different phrase for each spell they had prepared at the start of the session, and correctly pronounce it while someone is hitting them.