It's been a couple years, but the last time my BF tried to fly Spirit to NY, the flight was cancelled while he was driving to the airport (wasn't a weather issue, as other flights to the same airport took off fine/on time). He didn't see the notification until after he'd parked and they were on their way to the terminal. Customer service counter was a sh/t show, as you would expect. IIRC, he just took the refund and booked United for a day or two later. After that, he said never again.
I think that's your biggest issue - if for some reason the flight is cancelled, getting rebooked is not as easy because they don't have as many flights as United.
With that said, EWR is a hot mess right now, too.
Might look into Jet Blue to JFK (if going out of IAH), but IDK how much you'd save.