I have no doubt that some of the time, things go well on Frontier. And, all this "I had a good flight," "I had a bad flight" is -- seriously -- not very indicative of how any one person's experience is going to be, whether it be Frontier, United, SWA or any other carrier.
Somewhat better, is to look at the complaint stats compiled by the FAA (or whatever government agency does it). And, last I saw, Frontier's record was not too stellar. I will say, I am not a fan. I chose Frontier many times BEFORE they switched to the ULLC model and was extremely happy. The six times I have flown with them after the ULLC switch, not so much. As I mentioned earlier (maybe on another thread), wife absolutely refuses to consider Frontier since the switch.
I will say, now that their financials are looking good and they are (so they say), going to restore some of employee benefits/pay scales that they took away a few years ago, there may be significant improvement in customer service. But the fact that they even acknowledge that they have "issues" with customer service tells you something.
And, the stories of folks whose flights were cancelled and then they had to wait DAYS, well, that would give me pause.