Just the usual yellow band lottery, dubious support, and the mere act of getting an invite. So, realistically, it's just the yellow. If there's nothing else you can avoid any dealings with them. I have a minor yellow band but isn't a deal breaker.Does OnePlus One currently have any problems/bugs or have they all been fixed?
Software is good - CM11S is fine in my experience, as are many other ROMs. Lollipop Alpha builds already showing promise in both speed and stability.
I'd say anyone looking to pick one up that it's still a good buy although there are plenty more options on the table of you're willing to pay more or spec down. And I guess that's why they're still in demand.