In 2+ years and a couple dozen F9 flights, I think I've only once had a significant schedule change, and that includes many flights >2 months. And I managed to parlay that into the day before which was better for me anyway.
I fly United a lot less these days, but 5-10 years ago the same question could be asked about their schedule. There were always schedule changes. The difference I guess is that on F9, you pretty much take it or leave it, and on United, they'll generally be able to get you something close-ish.
If this is a problem for you, here would be my strategy:
Book your F9 flight.
Then also book a backup on United.
Most likely you'll just end up cancelling the United flight, and getting a credit which you can then use to "protect" your next F9 flight.
Some will say that's hassle. Sure. But everything about Frontier is a hassle to some degree, lol. But oh the savings....