The main area where the clone-makers skimp is on memory speed grade, and consequently they tend to slow the clocks down by 10% or so to improve their yields. This is irrelevant to MAME, which has very low video card demands in the first place. The one other notable place where I have seen 2nd tier vendors skimp, however, is in the quality of the TV-output chip. This has been a problem with the Nvidia-based cards, but I can't speak for recent ATI-based cards.
Sapphire supposedly follows the ATI reference designs very closely, so they are generally okay, but I would be leary of the others without detailed research on a specific card.