I find the value of interlining extremely overstated. The legacies don't really like to put you on someone else's flight and they aren't obligated to. They usually only do it for high level elites or high fare passengers.

However the impact of any delay or disruption will be amplified on a ULCC like Frontier due to their low frequency of operation in most markets..

If your flight is cancelled your options are to wait or buy a ticket on another airline at walkup prices. Either option may cost you through additional hotel expense or air fare. If you fly a lot you can save a fair amount of money and in the unlikely event your flight is too delayed you can buy a ticket with the savings or you can possibly purchase travel insurance or you may have some coverage under a credit card, but that usually comes with a lot of strings.

Cancellation rates for pretty much all airlines are under 2%. The latest DOT report lists Frontier as cancelling 1.2% of it's flights in February.

Another issue many people have with ULCCs is the limitation of what you can carry on. Frontier limits you to a "personal Item" such as a purse, small bag or backpack which will fit under the set. There are specific sizes specified but in my experience Frontier is not too strict as long as you aren't ridiculous. You do not have to put the bag under the seat and since the charge for "Carry-on bags" aka roller bags they overhead bins are usually pretty empty. It is also cheaper to check a bag especially if you pay in advance.