Motherboard issues?

How common or luck of the draw are motherboard issues? I received a brand new SD2 after having Z banding issues with the first SD I purchased and within 3 months or so of pretty good results I started having Y motor issues. When I sent the new machine back for repairs they said the motherboard broke and everything else was fine. So they replaced the motherboard and now I'm expecting it back in a few days. What causes motherboard damage and is it often just shit luck?