As an example, I'm running a 5e game for a group new to 5e. The one player who isn't new to 5e considered a rogue with Observant. The problem is that the intention of this game is to let the new players get a good feel for how 5e plays. We're playing the first two adventures in Tales From the Yawning Portal, which are trap filled dungeon adventures. Observant means he's almost never going to miss a trap, since none of them have a DC higher than his passive perception would be with Observant. Now, I consider traps as something you actually have to make a bit of effort to avoid as a basic part of the game experience. Taking that feat would essentially mean that the group is deprived of that particular element of play. We went back and forth discussing it, he mentioning that spotting the trap doesn't necessarily mean you know how to disarm it (true), etc. But I'm looking at the adventure and thinking that as a player I'd have a better time actually having the traps matter. What I finally decided is that if he did want to play that character (he had other ideas, and that isn't the one he went with) we'd have a talk with the rest of the group first. I'd explain the situation, let them know how their experience would change if this feat were in the game, and let them decide what they wanted to do.
At the risk of derailing... this is basically like me saying to a player "Yeah, I know our next campaign is Storm King's Thunder, but I'm not comfortable that your PC knows how to speak Giant, knows spells like Suggestion that can take them out easily, Fireball that will kill all their mooks, and so on... it's going to make your life too easy and I really want to let you experience just how amazing Giants are..."
My point being, while the DM can of course do what they want and stop players taking certain options, at some point a good DM realises that players like to chose options that are tailored for your campaign, and that's a good thing, it means they are paying attention and planning ahead. It's not like 5e is terribly broken like 3.5 was, and you can always adjust the adventure a bit to accommodate things like super-powered PC's, if you feel it's too bland as written.