The biggest problem I have with low-cost carriers like Frontier and Spirit is that they have a more limited flight network and don't have interline agreements with other carriers. So if your flight is cancelled or severely delayed due to weather, mechanical issues, crew issues, etc, the best they can do is book you on their next flight with available seats, which could be a day delay or more. If you value time on your vacations, I would recommend spending a bit more for a major carrier, which all have large hub-and-spoke models for more connection options and can re-book you on another airline if there are any major issues.