I tend to lump Frontier, Spirit, and Allegiant together as airlines I would never consider unless all other options were completely crazy. The main reason is that, where I am at least, all these airlines have at most two and usually just one flight to Orlando per day. So if anything happens to halt that flight, it's over. My only chance would be to get a seat on the second flight, assuming there even is one, and it's far more likely that I will get bumped to tomorrow or even two days back. I never have enough flexibility in my holiday schedule to absorb that kind of issue. My recent trips all were on Southwest, which seems far more reliable than the other three; if something happens, I'd have a pretty good chance of making it to MCO via another connection and losing not more than a few hours.
Now a lot of folks will tell you that they've used Frontier or Spirit for a long time, on various routes, and nothing much has ever gone wrong. (I am less familiar with Allegiant, which serves Orlando/Sanford a few times a week from Toledo OH.) To me, those airlines represent a major gamble that I am not willing to take. They are unquestionably cheap, sometimes much more so than the competition, but in my opinion their approach to customer service is unacceptable and I won't subject myself to it.