I have just worked an album project recorded with Cubase 5, then transferred into Cubase 8.5 Pro. Of the Steinberg plugs used in the project files, De-esser and VST Dynamics didn’t transfer. All others worked fine including Reverb A.
Halion One presets had to be re-assigned the equivalent sounds in Halion Sonic SE - but the same library patches are there.
On this project there was only a small selection of third party VStis. I asked that the client freeze those before exporting and they came across fine too.
All volume/pan/mute automation transferred. I did null tests from the Cubase 5 rough mixes. I imported the mix into a new stereo track in C8.5 with the project, then reversed the channel phase. When perfectly lined up with the unmixed version and identical in volume, they should null out, that is cancel each other perfectly. This wasn’t quite the case as I discovered that Cloner FX remained. Probably Cloner is free running in it’s detuning and timing cycle and thus will be different each time a mix is rendered. All else nulled out to silence.
My experience was largely positive. Hope this helps.