I am running a proxmox server hosting 10 servers and 30ish clients without subscription, Users are annoyed by the message popup, other then that everything works. I delay updates a bit to prevent issues.
I will license the server, but have had a year of trouble-free running.
No functionality is limited.
Given your mentioning of multi-core servers, I presume you mean multi-cpu ? If not, if you have a single socket in use, even if it has 20 cores, that counts as one.
This scenario would be just short of € 1600,- on their premium supportplan. That, to me, is not expensive.
If, otoh, you have 4 sockets with 10 cores, this is not funny, granted.
Besides an annoying popup, there is not really any issue with running the totally free an unsupported version, and there are howto’s around on how to get rid of that popup.