I personally am not fond of a purebred Friesian. They all look alike and most move funny and either are pulling themselves around or have the hind step to far back. They look very akward to me.
But I'm not a fan of them under saddle, I've seen very few that I like - as you said, they pull themselves around on their shoulders, and the hind legs are a mile behind. Extensions they just go higher.
I think if you want a friesian as a saddle prospect, you'd get one for loving the breed, not for wanting to be competitive in a discipline other than in harness.
As to the crosses, I have yet to see one that I like. Some of the friesian/wb/tb crosses look ok, but still nothing that I'd jump out of my skin and give my right arm for.