Many times, longhaul delays at SFO aren't related to operation of aircraft, but getting aircraft moved, cleaned, and catered on time. Most of my int'l delays have been due to "late arrival" of aircraft, yet they arrived the night before and just weren't cleaned or catered on time and/or weren't brought to the gate from remote parking on time.

I've been burned a few times by having separate tickets, so now I'm more cautious and have longer layovers. Missing the first flight of a second ticket can be expensive.