Microsoft's migration paths (GP to Business Central, NAV to Business Central, AX to Dynamics 365 Finance & Operations) handle the core data model well: standard chart of accounts, standard entities, standard relationships. Years of customization on top of any of those systems, custom fields, non-standard workflows, add-ons from a reseller, usually aren't covered by the standard tooling at all.
The same gap shows up migrating in from a different CRM, like Salesforce, into Dynamics 365's sales module: contact and deal history map conceptually but rarely field-for-field, and custom fields built up over years of using the old system don't have an obvious home in Dynamics 365's defaults.