IMO, the chief difficulty with Frontier (aside from the need to know how to pack light!), is that their new reduced-weight standard seats are rather uncomfortable. They do not recline, and they feel like sling-seat lawn chairs to sit in, though they are much sturdier, of course. They also have gone with the tiny tray table option, it's big enough for a drink and a small snack, but it won't accommodate a laptop (it is approximately the size of a man's large shoe, squared off.) For me, that means that Frontier is fine for short hops, but if I were to take a long-haul with them these days, I'd bring something soft to sit on. (They do have some "premium" seats available for a $45 per-segment fee; these recline, are normally padded for aircraft seats, and have extra legroom.)
Note that there is a very large difference between Frontier and rock-bottom budget airlines such as Allegiant and Spirit: the fleet. Frontier was a full-service carrier until a couple of years ago, and the fleet is very new, as commercial fleets go. They also still have full maintenance facilities in DEN, and they don't cheap out on upkeep. IMO, that counts for a lot. I feel fine about flying them as long as I don't have a need for a lot of baggage, as their bag fees add up very quickly.