The only example I can think of where errata effectively made it a new card there is the one you use as an example, Hot Lead Flyin'. In my opinion there is no way that Unprepared is as game warping as that became. If you weren't running it yourself, you had pretty much no chance in a shootout against an opponent abusing it.
I'm not someone who thinks the errata to Paralysis Mark made it useless, but I know many do. You didn't even need to leave your home and get in a shootout to shut a dude down with old Paralysis Mark, and it was very easy to lock down many decks once you got a couple in play. It was massively game warping, far more so than Unprepared.
I agree it's a powerful card, and would still be great if it either lost the bullet penalty, cost 1 GR to play, or only hit a limited number of attachments. For me, it definitely isn't anywhere near as format warping as the cards mentioned above were, as it don't cause me massive headaches trying to figure out decks that I find fun to play that also stand a chance of winning a game. I can understand that if you like decks based around building up one or two tower dudes, or that rely on attachments for Cheatin' Resolutions or shootout tricks, it can be really annoying to have to face.