It is, but when it's order dependent, you have to sort the polygons from back to front to determine what order to blend them in (and which need to actually be blended), which is usually done on the CPU.
Now, order independent transparency would mean this wasn't a problem, but I've yet to see anyone implement this or even a GPU based polygon sort on the Wii U, so I'm not sure how feasible it is.
Transparency has actually been a large problem for games for quite some time, which is why you never see see-through glass houses in video games for example.