Many different opinions, while the GUS can certainly be used for "hucking waterfalls and big water", there is nothing wrong with learning in this boat or taking it down class I or class V.
As a general rule, the boat will be a little bit easier to roll given the mostly displacement hull, at least that is what I notice since it does not have the edges of a playboat. It does not have a not have a planing hull (flat bottom) and that will teach you about secondary stability and bracing.
I think you can easily progress in this boat as you could in a playboat/river runner. If your looking for comfort I think creekboats are generally more comfortable than some of the playboats as they usually have more volume and room. As far as being comfortable in rapids, most people take a bigger boat when they are stepping it up to have more control.
I think your buddies comment comes from most people's progression of wanting to learn how to playboat/downriver play before you will want to learn how to creek, which would lend itself to learning in a boat like that. I know boaters that creeked before they ever got in a playboat so it varies.
Hope this helps.....get a boat you feel comfortable in and paddling, then worry about the rest later once your hooked.
*****Obviously by my avatar I have a GUS and love it, but its one arrow in the quiver of boats for a gear addicted person like myself. I wish I was sponsored by LL and could offer that as a disclaimer