I can only tell based on my own experience.
Almost all visualizations worked as is. Only those which used X-PACK features obviously failed and I had to migrate them (for example dashboard drill down is not in Kibana OSS version, so I had to replace it with TSVBs which have links to the drilled down dashboard with automatic filter applying.
Security is a little more painful point as it is all under X-PACK in Elastic and you have to switch to OD security plugin. I just redefined the users, roles and tenants (a feature covered by workspaces concept in X-PACK). Also, Index Life Cycle Management can’t be 100% automated in OD (at least I did not find how, this is something I would really recommend to security plugin owners in OD to handle ASAP). See Error while rollover action. How to create rollover alias? for the respective discussion.
The rest was pretty much smooth.