I was pleasantly surprised when my Frontier flight was canceled. This happened in September, 2016.
RSW-CLE, the flight was delayed for many hours and then finally canceled, after it was too late to get a flight home on another airline. The cancellation was not weather-related.
At the ticket counter they handed me a letter which gave me two options - I could get a refund of my ticket fee and find my own way home, or they would book a flight for me on another airline and pay up to $400 of the cost.
I chose the second option. This was not an interline agreement, I had to call a phone number to a Frontier travel agent in India who took over an hour but finally figured it out and we had a flight home the next morning on American Airlines. It was slightly over $400 each but they paid the full fare for us (another family's flight home was $500 each and they had to pay the extra $100 themselves).
Frontier also gave us vouchers for a meal at the airport, and they paid for a hotel room.
I was amazed, I was expecting them to tell us they would have space on another Frontier flight home in a couple of days.
My only complaint was that I had purchased "The Works" package which included baggage fees, but I had to pay AA for our suitcases. But it could have been much worse.