First, as someone with relatively long thighs, I literally don't fit in their sardine can seats. But more relevant to most people, while things may be OK if everything goes perfectly and nothing is delayed or cancelled, you are completely SOL with Spirit/Frontier if something goes wrong (and "something" may just be they themselves decide to cancel an undersold flight at the last minute). It's nearly impossible to get someone to talk to, I feel like the employees know how shitty their companies are so they all have an attitude like they DGAF, and it's a mad (expensive) scramble to find alternative arrangements at the last minute.
I've never had as abysmal experiences as I've had on Frontier compared to any other airline.