Suppose it's cancelled. Does Frontier have another flight that arrives into SFO in time for your TPAC flight (including retrieving bags and checking in for the flight to Asia)?

This will be separate tickets so the risk is on you if you miss the (I assume) UA flight to Asia. OTOH, if both tickets are UA, they're more likely (but not required) to work with you if something goes wrong.

Since Frontier is a LCC, I assume it doesn't interline.