@ScanX As with many Google software/service launches, I imagine it is at least to some extent a matter of trying to manage the variables in play. If this follows the typical pattern it will only be a Pixel exclusive for a short while before rolling out more broadly.
Basically it's for quality control purposes, however I own a Pixel 4 XL and I can say that the quality is awesome. it's super fast and I'm able to use my PS4, Xbox one, and Stadia controller with no input lag at all.
Got it working using the chrome browser on my Galaxy S9+ with a PS4 controller connected via bluetooth. You just have to make sure that desktop view is enabled in the browser options settings.