I think people may be getting caught up my calling a roux sauce grainy. When I say that, I mean it’s a fundamental property of those cheese sauces. They’re all grainy in comparison to something made using other emulsifiers. Even the best roux-based cheese sauce isn’t that smooth. Most of the time, like in baked pasta dish, it’s totally fine. I just don’t care for them outside of that specific use.
The texture doesn’t seem to bother me in something like a creme pat.