That is literally like the age old scenario where you take over an older IT firms network and need server access and have to hound everyone and their mother for the passwords lol.Funny you say that. We just moved onto O365 in a Hybrid environment. I assumed this would cause us less hassle but sometimes it can be a pain in the ass. We had issues with public folders appearing and disappearing on desktops and Microsoft literally had no answer for us. Literally 2 months passed and they still don't know the issue. Tried blaming the inhouse desktops but I proved them wrong by doing a clean win 10 install O2016 and the issue persisted. Of course, you don't have any real access to their hosted side so sometimes it makes things difficult.
It's nice in a way as uptime has been pretty consistent but we had to kick up the internet. We are trying to move file sharing onto the cloud and get rid of some of the inhouse servers. We used Barracuda SSL VPN for a while but for file sharing it is literally the worst system.
The public folders is almost always a permissions issue and when its a third party handling it I guess it could actually be more of a pain. Microsoft is the worst when it comes to support...They always have been that way.
Your right though uptime is def increased in the hybrid and hosted networks because of the multiple forms of redundancy. Its crazy to think that at one point "the cloud" didnt exist and all there was as you said were VPN's whether it be ipsec or SSL. If you had no internal network access, you had no shared files or folders.
I was using barracuda boxes for mail filtering back in the day and the rules for mail flow were so cryptic so I cant even imagine the file sharing. I guess it calls for patience more than anything.
Clearly you know your stuff and like what you do.