Obvious answer: We endeavor to fix the bugs before releasing the beta. That said, there's always reality:
I'd say the most common problem encountered is features not working (or not working the way you expect). You might use a feature in a way that we didn't anticipate, and we make a change that prevents it from working as before. This always happens.
Other issues could be downtime or loss of availability. Sometimes the new code will crash on your data, leaving your site unavailable until we fix the bug. This usually happens to somebody, but not most people.
For On Demand we keep a series of backups of everyone's database (and you should do this too for Licensed), so if the beta really does eat your data, we can roll back. You might lose a few days of data, but not everything. I'm not aware of any instance of this actually happening.
In my experience, the beta releases may have more bugs than stable releases, but they aren't more severe. The majority of the data corruption bugs I'm familiar with were all discovered long after release.