Beyond the grounding/connector issues (which I've never really had issues with, at least not worse than other games), the main thing is that they're just not designed to be worked on. Lots of hard to find chips, no test points or diagnostic leds/tests. Every failure in the game can have tons of different failure points, with no easy way to figure out which one is the issue. They have an 'interconnect' (similar to williams) between the mpu and driver board, but while williams' sends the actual address bus across (so any interconnect issues will basically cause the game to crash), gottlieb just sends data across to latches on the driver board, so a flaky connection will leave the game running but solenoids/lamps can just wind up being stuck on/off due to flaky connections. The boards weren't designed with much future planning in mind. The driver can control very few solenoids, the sound board only had 16 possible sounds, etc. When that became an issue in newer games, they came up with creative hacks to work around it which add even more failure points. Their electrical engineers seemed to just design for "get it working right now", with no thought for reliability down the road or future issues.

Even worse overall. System 1's OS roms were hardcoded into its six (!) CPU chips which are soldered directly to the board. The games can even boot and run tests without the rom installed. Horribly un-future-proof. The grounding issues are even worse as a bad ground can lock on a solenoid, and on system 1 that can even come back and damage one of the CPUs controlling that solenoid.

I don't think they're nearly as bad as many people say, either, but having worked on a ton of every system pretty much, I still find gottliebs to be a bigger pain when there is an issue.

Black Hole seems to be its own special thing for sure. Although mine has been solid across three moves now that I got the kinks worked out.

Wait, they have? I thought they were discontinued and running low on/out of stock

Even worse, system 1s give an artificial pause before booting just to make you feel like it's doing something