I have just installed Nextcloud 15 on a (shared hosting) server that has a max PHP memory limit of 192MB.
So far it seems to be running and “The PHP memory limit is below the recommended value of 512MB” is the only warning I can see in Administration >> Overview.
What disadvantages will I be facing with that limitation?
I think nothing more than less performance compared to more available RAM for PHP and probably some swapping. It could still be enough to feel fast enough, if there are not many users using your NC server.
It will only be 2 to 5 regular users. It’s only a warning and obviously no show-stopper, but since I could not find any clues in the NC documentation about it, I was concerned that certain functionality would not be available or limited.
Well, certain apps might not run properly. Collabora/ Onlyoffice (if run on the same machine) require 2GB RAM at least. I can image that Talk will also require a little more RAM compared to simple file sharing and so on. It really depends on the intensity of usage. If you notice really bad performance or limitations regarding the usage of some apps, you can still think about upgrading your contract to a VM with more RAM 
By the way: I couldn’t imagine there are still shared hosted servers with less RAM than 512 MB. Aren’t there better hosters with a little more RAM for the same price?
As it was said, with some apps you might get problems. But if you don’t need them…
What could be a bit of an issues are upgrades. If you can’t run the occ command for upgrades you must hope that the memory and runtime limits are enough to run the whole upgrade. Especially when the database grows larger, this can get tricky. Unfortunately, I can’t give you more detailed numbers. You could install a NC 13, put a few users and files, put some contacts and shares in the database and see if an upgrade to NC 14 runs fine. If not, I wouldn’t even start with such a setup but even when you succeed be prepared to hit the limit some time.
None of the web hosting offerings of my preferred ISP (Hetzner) features 512MB RAM and the two instances of NC that I have installed just run off regular web spaces which I have access to and do not cause me any additional costs (thought it’s a wasted opportunity with more than 2 x 40GB unused space and unlimited transfer included).
Hetzner is offering manged Nextcloud instances and I’m tempted to go for the smallest package, but if the two NC instances working in federation will do just fine, I’d rather keep it like that.
It all comes down to the two limitations:
I think I’ll try a third installation of NC 14 and try to upgrade to NC 15 to see if/how this works out.
I just tried the upgrade from a fresh installation of NC 14.0.4 to NC 15 and it worked, but I did not add any data / users / files / contacts / shares beforehand.
I’m not sure what will be the bigger problem for maintaining such an instance, the inability to use the occ command or the limited RAM.
The Nexcloud offerings from my ISP obviously also do not have shell access, so the occ command can’t be used with those either, not sure how relevant that is considering that it is somewhat managed with automatic updates.
Personally, I wouldn’t waste my time with such a setup. It’s very likely that you run into problems at some point and it is likely that it happens when you don’t expect it. I generally have doubts about hosted environments, but if you are way of the recommended settings from the beginning, what do you expect? If you need a really cheap solution, consider using a small ARM device on your home internet connection or spend a bit more money on a vserver. You will spend a few more bugs per year but save a lot of time.