As an integrator, I have been an advanced certified individual in the OnSSI software since NetDVMS. I have worked with almost every version since then up until now. The difference between Ocularis 4.x and 5.x is minimal at the Base level. But the recording component on the back end of 5.x is about 5-7 years behind the version they were using from Milestone. In a word, it's horrible.
Camera licensing just "disappearing" is nothing new for me. It happens a LOT in RC-E. But I think that is a fault of the milestone backend, and not the OnSSI Base since most of these problems come from the Milestone side.
The client does not seem to have changed all that much. If you are having a problem with the client, I suggest you try a version of the client that is a little bit older. I use 3.x client and have excellent results across the board. Well....as excellent as you can expect from OnSSI software.
My suggestion to all of my customers running any pre-5.x version of OnSSI is to wait to upgrade it until I feel confident that it will not break anything. And I continue to evaluate OnSSI's new version myself. Having recently taken the advanced course in OnSSI and having played with their new SeeTec recorder, i'm not recommending any upgrades at this time. As such, they can not continue to buy licenses for their Milestone backend unfortunately, but luckily for me, all of my OnSSI customers like to keep extra licenses on hand. So we are not in a bind to move forward with the new version yet. Not to mention there are MANY features in the milestone version that would no longer function on a new OnSSI SeeTec recorder.
That said, despite what OnSSI suggests, I would upgrade the base to 5.x, but leave all of the rest of the recorders running the milestone backend. This should not cause to much headache as any pre-existing issues would still be in the recorder, but, hopefully, there will not be any or many new bugs in the new base version. This will give you the ability to test the new recorder on a test server and evaluate it properly before putting it into production.
Best of luck to you.