Mainly his bloated contract.
Coincidentally Allen is the coach who drafted Carr in 2014, so there is a small connection.
He is an accurate passer (if you look up the stats) , doesn't have Winston's injury history and threw less picks over his time with the Raiders, is younger than Dalton and more mobile than him, is a 3-time pro bowler from a team that was/is a dumpster fire-so I mean there are definitely some positives.
I think with a stable o-line, right OC drawing up plays, maybe another WR to replace MT (if he leaves) and a solid downhill bruising back, we could be on to something if we scoop him up.
At the end of the day, he could be a better fit vs. Dalton (who can be his backup) and Winston (who will probably bounce).
Again, the contract is the key in all of this. I'm not a contract guy, so I am not sure how/if we can afford him.
EDIT: I see that we wouldn't have to absorb his contract above, so that is a plus. We'd have to offer-up picks/trades I suppose.