I have 3 Google Stadia (Chromecasts) in my home. With only the router (Deco M4R) connected with beamforming off I get decent (but not great) performance from all 3 at the same time. If beamforming is on then the service becomes unstable. The only problem with using just the router is that one of the Chromecasts can barely get 5ghz and will often switch to 2.4ghz making it unplayable. However, when I connect the satellites (Deco m3w) all three become unstable again. 

Any updates with firmware soon?