...I asked in MacRumors forum time ago, and the BIG problem was exposed (because all professionals using this kind of media know):
sd cards are probably the most unreliable storage devices in the long term. They're used for recording data (as photos) and taking the data out of it in a short while. Nobody stores sd cards as back-storage kept in a drawer, and that would be quite convenient related to space!
Choosing the less reliable method is not the best idea for security back-ups.
Of course, there're a lot of interesting alternative uses for sd-cards... but I would keep Time Machine (and other copy) in a safer place (common HDs are better and cheaper)
AND I WOULD INCLUDE SD-CARD IN TIME MACHINE BACK:UP, to avoid the crying at (sure) first sd.card corruption.