ISPConfig 3 is very stable controlpaneland it does not has alot of bugs. Otherwise it would not be the most used open source hosting control panel with several 10 000 of servers. The bugs that are there are listed in the bugtracker and that are not many as especially no severe bugs.
The recommneded Linux distribution for ISPConfig is Debian Linux. You find this recommendation all over the forum and in the FAQ. If you choose another Linux distribution, it will work on that fine as well but the setup will be more complicated.
If some software packages are not available for a specific Linux distribution, the thats not ispconfigs fault. The compilation is described in the perfect setup guides, but thats not nescessarily a task for Linux beginners. So better use Debian and you wont get any problems.
I really didn't see anything that would make it a must when compared to 2.X
A lot of more features, you seem to have not explored ispconfig 3's capabilities yet. For example multiserver support to manage complete server farms, extended monitoring, virtual mailusers etc.