I'd say the main drawback of using pixmaps is that they're *very* slow. If you draw a fullscreen pixmap each frame you will suffer heavy fps loses in most machines.
I would use pixmaps for images that, either aren't going to be drawn each frame, or are small and few but they need to be changed realtime (and a lot of times, maybe one per frame).
Or else, use them at specific times - for example, converting a TImage to TPixmap, modifying it, and then back to TImage - it's "slow" too but only once, and not as slow as drawing a pixmap every frame.
I would use pixmaps for images that, either aren't going to be drawn each frame, or are small and few but they need to be changed realtime (and a lot of times, maybe one per frame).
Or else, use them at specific times - for example, converting a TImage to TPixmap, modifying it, and then back to TImage - it's "slow" too but only once, and not as slow as drawing a pixmap every frame.