Newer boards can be a lot harder to fix due to miniaturization and big surface mount chips. What happens when you finally troubleshoot the problem and it is an 80 pin surface mount chip. I'd say about 1 percent of arcade game guys can solder well enough to replace one of those.
Pre-Jamma stuff also tends to be easier to diagnose, since most faulty JAMMA boards I have seen do one of two things, either display nothing, or display garbage. While most classic boards tend to still do stuff when they have problems.
Faulty classic boards I have owned have done all the following.
Played the game with garbled sprites, and/or backgrounds (lots of games)
Had something missing from the display (score, etc) (Rally-X before I began repairs)
Had missing sprites (Space Pilot)
Had funky vectors (Star Wars)
Added strange things to the background. (Circus Charlie)
Had incorrect colors (Rally-X)
Spelled EVERYTHING wrong (Sega Turbo).
Incorrectly scaled objects (Sega Turbo).
Bad sounding audio (A LOT of boards)
Missing shots (Space Firebird)
Booted to garbage or blank screen (Track & Field and a few other Konami titles, only a SMALL percentage of the defective boards I have had)
Meanwhile as for JAMMA boards.
90+ percent of the defective ones have booted to garbage or not displayed anything.
One (TMNT) constantly reset after self test.
One (Last Duel) had missing audio.