Hmmm, that definitely seems to quality as a pattern...which conforms to Cakewalk support's experience that the greatest number of stability issues relate to third-party plug-ins.
Unfortunately there are many ways good plugs can go bad, the main one being the existence of 32- and 64-bit plug-ins in the same project. Don't know if that's the situation, but bridging is a kludge. One of the better decisions PreSonus made when they introduced the 64-bit version of Studio One was to increase stability by not including
any way to run 32-bit plug-ins. When someone at PreSonuSphere asked Wolfgang what he recommended for running 32-bit plug-ins in SOP, his response was "Tell the manufacturer to make a 64-bit version."
Again, I don't know much about your system or even whether you're running 32-bit or 64-bit, but I will say that when I made the decision to use 32-bit plug-ins only as a last resort, I pretty much stopped having issues with SONAR once I also stopped using an audio interface with problematic drivers. Since then crashes are virtually non-existent, and I use SONAR almost every day.
Furthermore, SONAR sticks very close to the VST spec, and some older VST2 plug-ins may have taken liberties that didn't matter prior to the spec change to VST3 but show up now. Of course, there are also plug-ins that weren't designed very rigorously in the first place. Sadly many of these are one-of-a-kind, cool plug-ins designed by mad scientists, and I've had to wave goodbye to several of them over the years.
As to UA, their attitude toward SONAR changed a while back and I think you'll find that current versions work well with SONAR...at least they do for me. But remember that some of those plugs use a lot of sample buffers, like the Manley Massive Passive.
[Just noticed
this thread, which expresses sentiments similar to mine]