I'm ex-Vera sorta, I went to ST in the middle. When I was on Vera it was just 1 lock and 1 thermostat. Ahh the simple days when everything just worked (ask little, you get a lot).
Then I moved to new house bought ST and a ton of Z-wave switches. I didn't have a lot to move over, so I cannot speak much to that transition. However ST outages/lag drove WAF into the dirt. When that got tolerable I found Lutron Radio Ra2 (primary lighting solved!), and HE for the remaining devices/sensors/colored light/etc.
Was replacing/moving all the device fun? No.
Has HE been all butterflies, and rainbows? No.
Are there things "that worked in XYZ" that don't work as well in HE (locks)? Yes.
Would I go back to ST or Vera over HE? No, between the device and the community I would not trade in my HE.
As for processing I have started using node-red, which I personally find "easier" to use for complex rules/flows once you get the hang of it. Having it run on a separate box has removed a load off the HE box and improved our overall "experience".
WAF is now off the charts and I only hear complaints when I am "tweaking" something and screw it up. However the bug has firmly bitten and I find new devices show up on a regular basis.
If I had to say 1 bad thing about HE and this community, it's the fact that it's brings so many new options to the table that I cannot keep up. My list of "I gotta try that"'s is getting longer and longer.