I think that's kind of rhe point of these sorts to demos to begin with.
The company says we're developing a product that, we are not ready to ship today, but will be this awesome. Give us some money and you can see how awesome it will be.
I generally assume that anything a company says about a product/service that is not shipping today is the best possible spin on the best version of what they'd like to sell. What you buy probably won't be what is shown as an early demo
Not a big gamer, myself, but it seems like Valve has done a lot of work to make many (most?) Steam games run well under Linux.
Drivers have come a long way, and a lot of things just work, but it can definitely depend on what hardware is in your system.