Square or "Box" ways, if sized properly, can carry heavier loads then any other type of the same size. That's the good news.
The bad news is they are more prone to "stick-slip" and are better suited for much slower motion speeds. There are exceptions like air bearings on box ways etc., but I doubt your mill has them installed on it.
There are "plastic" surfaces that ride on the ways such as Rulon etc. that are an improvement over metal on metal surfaces.
All said, for a typical milling machine, IMHO box ways are fine.