I have cork roadbed (25 years old) and have no hands-on experience with foam roadbed, Ross or otherwise, but have known someone who has used a foam roadbed in HO scale, but I don't know the brand.
It was fine at first, but over several years it began to lose its integrity and shrink or sag under the ballast. I think that he replaces it as time goes by with cork.
Which has nothing known to do with Ross products (love their track, etc.), including the roadbed...but, it does have to do with foam. Some foam, at least.
Cork is stable and predictable; some say that it "gets brittle over time"; it does, I guess, but I don't care, as I don't exactly go move my layout or track around just for fun. It lies there undisturbed, bothering no one, and not shrinking or sagging.
I have no axe to grind; just relating facts.