If you pay by credit card and it's a domestic flight, the financial risk is fairly small. However, if your ticketing airline goes belly up, there is the risk of all the seats being taken on competing airlines if you're flying during a popular holiday period or on a route with limited competition.
So if you MUST get someplace at a specific time, why take the chance on US or UA
If time isn't important, then book on US or UA
and maybe you'll get to spend a night in some exciting hub airport in the middle of winter.
Seriously, now ... US is a bigger risk.