Be sure you understand ADFS and, if you decide to go with it, what you get and don't get with it. Your users will want it.
ADFS was sold to overcome the issues with entering passwords in all your clients (Outlook, Skype for Business, OneDrive) but, if your password changes, you do have to manually update passwords in all of these clients.
Also, this does not get you direct access to the 365 portal or SharePoint without modifying links to use Federated Services.
This becomes an issue if you use generic accounts for the shop. It will work if each of your users are required to use their own ID. Also, note there is a cost for the Azure instance of ADFS (servers, compute, network, VPN).
I have used it for nine years.
Deployment went very smoothly even in its infancy. My most recent deployment went off without issue.
Others report stability issues and there are often warnings in the console but the service has been flawless for us.
We have not encountered any scalability issues. We have probably increased our user base 20% since inception.
Customer Service:
For general support, it is offshored and you open a ticket in the console. It works pretty well. For how-to's or best practices, you will need to have a good partner.
Technical Support:
Generally, the technician has a good idea what you are trying to do. There is often a handoff to a group that can actually DO the work.
Our vendor assisted with setup using the Jump Start program. There are complexities with certificates, hybrid, etc., but overall, it went well.
A vendor team implemented it. I would rate them 10 of 10 at the time. Since then, our resource has moved on but they still have a pretty good skillset.
I don't have my spreadsheet handy but the ROI was there. Microsoft also provided significant incentives to make the move.
If it is an in-person query or reference, I do let them know the deal I got, as well as the approach that got me there.
Because of the significant investment we already had in Microsoft, this made the most sense; thus, I did not evaluate competitors.
You may need ADFS; you may need a hybrid environment. Not everything necessarily goes away. Also, user management continues; it just moves to a new place (hosted/the cloud).