The biggest problem with Stadia was not the technical implementation but the business model: you had to pay for both a subscription to use the service and additionally a license to play particular games on the service (though there were also some free games). And of all the companies to even attempt such a business model, it is harder to think of a company that had the least chance of making it work than Google because almost no one believed that the licenses they paid for would be good for anything in a couple of years. In fairness, Google did refund these purchases when it shut down Stadia, and this was absolutely the right call, but it is also befuddling because, if they had been planning on doing this anyway, they could have told everyone at the beginning and made people a lot less wary of spending money on Stadia!

Edit: The above was my understanding at the time, but the responses below, which I appreciate, would seem to indicate that this understanding was incorrect.