Failure is the one that always puts me off... At least with United, there's a good chance they can get a broken plane running again, or swap in a different airframe, within a reasonable number of hours. For example, my last flight to Puerto Rico was delayed by ~5 hours, due to a nose gear problem. They eventually swapped air frames around, giving us one that was scheduled for the late day flight, and got our air frame fixed in time for that later flight.

Spirit or another super-low-cost? They don't have the extra air frames and number of flights to do that. You get to wait even longer, losing valuable vacation days (or missing work meetings).