Also your load balancer stack needs to support it, which is becoming less of a problem these days but there's still some stragglers. You could also end up with an imbalance where some servers have a lot of connections and others have few.
Yes that's some genuine concern. Though on B2B and B2C SaaS where you got paying users, I think each of them deserve a websocket, and it could* alleviate some headaches for developers.
Needs a fairly capable server though, I suppose. One that can handle lots and lots of open websocket connections.