Things like vibration and Pogo oscillation (https://en.wikipedia.org/wiki/Pogo_oscillation). All the engines collectively have about twice the thrust of a Saturn S-IC first stage (on the Saturn V rocket) and they're all going to be trying to shake the crap out of each other just due to thrust oscillations. If they hit a resonant frequency of the rocket it could shake the rocket apart. So, you can have 33 engines that all individually ignite properly on the pad and are all well-built to tolerances so that they function, but when you put the whole vehicle together and have all 33 of them pushing against the same airframe they could spectacularly fail.
Yes but the reliability of modern car engines is a minor miracle, achieved through over a century of incredible engineering effort. Also remember that production lines always have yield, which means that not everything that rolls off it will actually work and the non-functional components are discarded. For example new chip production fabs very often have low yields, sometimes even the majority of chips don't work.
