Nebulous
Legend
It's totally the DM's call about which feats, if any, are allowed. However, nerfing things mid-game is usually considered bad etiquette to be avoided in all but the most extreme situations.
On a personal note, if a DM is worried about a feat making you un-surpriseable...well...that sort of implies a DM who is wedded to a certain type of "antagonistic DM vs. players" style. That style can be fun, but it's not for everyone.
I'm not a fan of changing stuff mid game unless we mutually agree it is "broken". I DO allow feats in my game, and I realize that by mid to high level the PCs are more powerful than most the stuff they fight, so then I have to start adding Monster Feats just so the baddies can keep up. Really, I wish there was a 5e supplement of monster feats by CR and Monster Type that you can just slap on to any creature.
Anyway, I'm not a fan of Alert's "never surprised". Never? EVER? So, is it like a Spidey Sense supernatural thing? A sniper couldn't hit you from hidden cover because you'd know the bullet was coming.