Why is it shitty?
Stop supporting Israel and meddling in the region, and suddenly there is no need for a deal to begin with.
Because it basically allows them carte Blanche in 10 years to build nukes
It actually is not that bad of a deal from nukes perspective. Iran is free from the deal in 10 years, but not free to build nukes since US and Europe, also free from the deal, will bring back sanctions in no time of even take military action if there's any indication that Iran is moving towards that direction. The issue with Iran deal is that US had a huge leverage with the sanctions system and could use it to negotiate something beyond nuclear matter. That might be a valid criticism, but it's also all in hindsight since you don't know how much Iran was willing to give up anyway. Also, US supporting Saudi Arabia's exploits in Yemen hardly has the moral high ground to stop Iran from meddling in the region.
Withdrawing from the deal is probably the worst option US has at this moment. Iran hardliners will block any new attempt on negotiations even if economy collapses (Rial has lost half of its value since November). Regular people will also be baffled how US can go back on a deal they signed themselves.