It happens infrequently. OP will be able to pick up the luggage if flying through SFO, guaranteed, cause you have to at the first point of arrival in the US.
If there are IRROPS you can speak to someone and offer some suggestions. Maybe UA is different, as I don't fly them much, but usually the airline gives you some options, or asks you to call in.

When I did a hidden-city flight I was never "forced" to take the re-routing; I just needed to come up with a creative way to explain why I needed that routing through a certain city (and given I missed the flight in the first place, I didn't have the mindpower to do that).

It is a probability game, but so is everything else (did you know flights crash? Or turn around mid-Pacific. That is real stuff that happens on a regular basis).

I think the OP is coming here for advice. Personally, it would be more helpful to tell OP the options and the risks of each, than to just say "oh, it is dangerous because this can happen", mentioning some event that is rare and dismissing all the benefits of trying it (and workarounds).

In fact, if OP actually listed the country than one can even more realistically plan for probability of not routing through SFO in IRROPS. Because for certain countries it is likely near nil. UA isn't just going to put OP on any other airline's flight, and UA has preferred partners it would re-route on.