We have GTB (single and multi-player) WMS (single and multi) BLY (single and multi) Chicago Coin and Zacarria (multi player only) EMs. None of them were working when we bought them, I'm not going to say they each got a full restoration but they were all thoroughly shopped and brought back to life, all, I feel, play as they should. It is true, they are all switches, coils, relays and springs inside and, aside from the Chicago Coin, I feel they're all just as easy to trouble shoot and bring back to life. I don't feel one is easier than the other. (though, I admit, a multi player takes longer to shop out than a single player, but once that part is done and the game has been properly set up and debugged, they're just as reliable)
All that being said, I feel GTB does have a slight mechanical advantage with some of their assemblies, they figured them out early and only improved upon their designs. While Bally suffers from their cheaper parts (such as lamp sockets or jones plugs) they did have the more innovative lift up motor board, more accessible score motor design and a few other previously mentioned things. Williams/Zaccaria sit somewhere in between GTB and Bally.
Gottlieb's real advantage, in my eyes, is the availability of replacement parts thanks to Steve Young and his Pinball Resource and other vendors that realized that there would be a market for replacement parts for these things and started stocking and creating quality parts.