And single gpu cards can also have problems with driver related performance, crashes, glitches etc same as multi gpu. The only real inherent multi gpu specific bug that comes to mind is micro stutter and that seems to be going the way of the dodo for amd come the end of July. I've ran 3 gpu's for well over a year and can count on one hand the amount of issues I've had, an issue with rage that affected most amd cards and 1 with battlefield 3 performance. Ever since the introduction of caps any real issues usually get sorted in a decent time frame. From what i've seen people really do a good job of going totally over the top blabbing on about multi gpu being ropey, oddly enough in practice for me at least its been fine.
A trend I'm noticing is that the people who are loudest about the failings of multi-GPU set ups are those who don't use them or haven't used them.
It's like the latency issue, the people being the loudest about it were people who didn't even have AMD cards, and they were so busy telling everyone who did have AMD cards, that their cards aren't smooth.
That isn't to say no one has had the issues, but rather how readily some people insist people are having issues when they're not.
I haven't had any of these latency issues in the massively widespread way people were making out, and I've had 2x 7950s for about 7 months now.
The main issue I did have was due to the spazzy boost BIOSes on my cards, which are easily sorted by flashing a different BIOS or just overclocking the card beyond the boost speeds.