That version of SignalR you are referring to is from .NET framework and really bolted on. That version of the framework had async/await in ASP.NET also bolted on, and the threads itself use a synchronisation context and locks.

This is mostly no longer the case in modern .NET. SignalR is a core part of the framework and completely rewritten.

In our case, Blazor Server has been very stable for us, also for multiple users.