Not op, but it's my understanding that they can blacklist pretty much the same sites as for exit relays. So your email client may stop working, Also search sites, social media, etc.
Obviously, anyone can blacklist anyone for any reason on the Internet and relay nodes' IPs are public. But do they?
I run a relay node on my personal server and never had any issues. But 1) I rarely browse the Internet from that IP and 2) it's in OVH so if it were blacklisted, it could be because of that.