FWIW, my experience doing this process for a ~130 person org last year was pretty painless compared to other Domain Claims I've initiated for other SAAS vendors (Docusign in particular), and MDM nightmares (expired JAMF certificates, I'm looking at you).

We had to do it as ppl had made personal Apple accounts using our domain, meaning if they logged in with such an account and left, their iPhone magically transformed into an expensive, elegant paperweight. Due to a setting in our previous MDM we were unable to migrate data cleanly using Apple Biz Manager without committing to use ABM as our MDM (we couldn't) so we told people to "move it yourself following these detailed instructions, otherwise it can't be migrated." Regarding personal data like health on company-managed devices, I certainly don't share that type of info with my employer, and make it clear to staff that it's not our responsibility to migrate such data.