My player has an invisible flying familiar that lasts indefinitely without concentration. It always passes stealth checks, never triggers pressure plates, and is small so it can fit in small spaces.
It’s possible, but it significantly reduces the variety of traps, so it becomes increasingly obvious that I’m just countering the PC, which is kind of unfun.
Every encounter suddenly starts to have boarded up windows, monsters with truesight, and stealthy NPCs ready to pounce. Etc.
This is the problem I have as well. With a group new to D&D, it’s not so bad - they’re likely to take creatures which are cool, but not necessarily stealthiest or most situationally-fitting. It’s okay to have the bandit ask “…wait, why is a raven this far into a giant underground mine?”
Then you have the veteran players who have invisible, indefinite, sometimes incorporeal familiars… the most egregious was one who would cast through his nigh-undetectable familiar, making many encounters moot as the familiar could just ping down stuff without ever being spotted in return.
