I think "design flaw" is a bit of an overstatement for some of the issues mentioned in this thread. Pinball is a game of mm. A slightly bent ball guide can make a big difference in how a ball reacts. No game is designed to have the ball go STDM from the pops 60% of the time. In any sort of hand assembled product like a pinball machine there may be variances and the machine has to be adjusted.
I get on location these things would be more frustrating than not, because you cannot adjust them yourselves, but it doesn't mean the overall design of the machine is flawed.