We migrated from 7.5 to 8.1 late last year (about 11k servers at the time).
We didn't use any of the migration assistant tools as they weren't fully baked yet. There were a lot of hurdles - tagging strategy, stubs to subscriptions, missing vRA plug-in for vRO, onboarding strategy, infr/k8s understanding and minimal documentation in general.
Most of our code lives on PowerShell script hosts (called by vRO), and so a lot of our vRO workflows could be re-used, maybe with a handful of tweaks here and there. But everything else was pretty much a rewrite. So it was a large effort but worth it in the end I think.
