Just because you hardly experience any problems doesn't mean they don't exist.
There are at least some problems with the online part. Even when I am host (which I am almost all the time since I have an 8Mbit down/1Mbit up connection), I still get the 'Server connection lost' message every now and then.
Must say I have never experienced any lag in any of the races I host though, and have had no complaints from others joining either.
So on my end I'd say it's very stable, but that does not mean that is also the case for the rest of the world ór that the problems other people are experiencing are their fault, I can't say that based on my experience only.