Its the combo of lousy mechanics and an air raid offense that I think is scaring people away. However, if one watches the tape, such as the LSU game, you'll see that he is spot-on with his throws even with that back leg waving about.
Mechanics on Mahomes are fine. He delivers the ball accurately. It's actually a plus that he can do without stepping into every throw in my book. In the NFL, you don't get perfect pockets. You will have fast 280 lb guys trying to kill you.