I have no idea what they are talking about. I have been daily driving GrapheneOS for almost two months now (coming from iPhone, but I have tested Pixels and Samsung phones on the side for a while) and there is no material difference in daily use from running the stock Pixel OS in daily use if you install Play (Services) and a bunch of apps. Of course, it is more secure and comes with no crap pre-installed, which is nice.

The only thing I have really found missing is Google Pay support for contactless payment (because Google doesn't want to allow GrapheneOS, but there are alternatives like Curve).