Efficient, has Hub and groups just like SignalIR does, and most importantly it does not force you to use a particular message format or JSON. You can adapt any protocol you like, and I use it for JSON-RPC
A decent implementation. It's very well documented and there is a blogpost explaining how to re-create it yourself. The disadvantage is that it forces you to use JSON and a spesific message format, which is not ideal if you have other clients that aren't .Net
This a server that does not rely on ASP.NET Core, it is just a websocket server and that's it - no REST, serving files, etc. This can be important if you are looking to add websocket connectivity to an application that has nothing to do with being a webserver, like a game. Starcraft uses websockets and protobuff for bot and AI development. This does work with unity.
There is currently a discussion about .Net Core compatiability, but I believe it's not quite done yet.