I took 763s out of SFO 5 times last year, 3X to ORD, 1x to MIA, 1x to JFK. Four of those flights were late departing due to mech issues.
Once to ORD, 3 hr delay due to replacing an engine, and I had to run to catch connect an int'l flight with only 5-10 mins to make it. To MIA, 1.5hr delay and again had to run. The JFK flight delay made me late for a meeting in NYC.

Fortunately, I was lucky and didn't miss any connections, but it was pretty nervewracking.

So 4 out 5 five mech delays... not good.