There are some great sound guys around, but it seems to me, it's one job where you don't HAVE to be good, especially if you stick to smaller/local gigs.
There's a local guy who gets loads of work round here and he just doesn't know what he's doing. He's really bad, but I've come across plenty of sub-par sound men too. The impression I get is that they've been running their gigs way too loud for years, which has f***ed their hearing, which leads to bad mixes.
I'm not making any generalisations about sound men, and I wouldn't claim to know that X% are good and Y% are crap, but, bad sound men do seem pretty common, at least around here.