> it's just moving, scaling, playing with opacity etc.. that's just updating the matrices once.

...any tiny change like this will trigger a redraw (e.g. the GPU doing work) that's not much different from a redraw in an immediate mode system.

At most the redraw can be restricted to a part of the visible UI, but here the question is whether such a 'local' redraw is actually any cheaper than just redrawing everything (since figuring out what needs to be redrawn might be more expensive than just rendering everything from scratch - YMMV of course).