I have a 256mb vps. It is running inspircd, atheme, znc, bitlbee, ejabberd, mumble, biboumi, and xmpp.js. Plus some non-communication stuff. I ssh in maybe once a year? I have to actively remember to go check on it because it never gives me problems.
I have a 2gb vps that runs synapse. There is always something wrong. Using all of my cpu. Using all of my ram. Using a ton of network. Generally slowing down everything else on the vps. Sometimes synapse crashes or doesn't come back up after an update or reboot. I gave up trying to dig through the gigabytes of log files that are produced. Why am I out of disk space? There is a 25gb table in postgresql for synapse that is... I dunno typing events or something. Don't worry though there is an api endpoint that might fix it. Good luck. I have 5 users.
A year (maybe 2) ago I gave up on federation and I'm just running synapse with sqlite. There are a lot less but not zero problems.
I've actually considered the idea of trying to set up bifrost to log into my xmpp server to use biboumi to connect to bitlbee running purple-matrix so I can join the matrix and synapse rooms on the matrix.org server using my matrix.org account. That fact that I've even thought that is insane. As crazy as it sounds I really do (sorta) actually believe that it might be more stable for me.