I am using WebSockets together with the High-level multiplayer which is working fine.
I am wondering wether it makes a difference to use rpc_unreliable instead of rpc because WebSockets already use TCP und thus are reliable.
I have also asked this question on Reddit: https://www.reddit.com/r/godot/comments/hgsprr/networking_ws_rpc/
I've tested it with Wireshark and both methods use TCP; so they are reliable.
WebSockets can never use UDP, so that was bound to happen :)