I went from version 9 to version 12 in conjunction with working on Canon DSLR and RED files. Without working on proxies this was necessary.
It is hard to say if Version 12 was more unstable as I was working with different video files - I do not use video card acceleration. I had more crashes but was this because I was working on more compressed or bigger files?
Many features in Vegas 10 to 12 I have never and will probably never use. 3D, close captioning. The stabilizer is not that useful - better off with deshaker, The LAB stuff is not that useful to me.
In fact now that I think of it without the support for the RED Scarlet files I probably wouldn't have felt the need to upgrade. The two added features that have been the most helpful are the synchronization alerts and the shift key for easy J,L edits.
As I have told Sony, stability and usability features always trump fancy new gizmo's for me. Implementing something like Vegesaur would be worth about 2 upgrades.
The biggest hardware difference between 9 and any version after 10 is that you will most likely have to change your video card unless you have one with newer CL and Open GL languages and architecture. In addition, the newer versions are computer invasive bowing to Gracenote monitoring technology (v12). Version 11 had severe problems with NewBlue add-ons. SVP 12 is more stable but, I think requires more HP.