If it's cheap enough it might still be worth it even with the extra wear and tear discussed ITT. Just be aware that if the SD dies, you could lose whatever saves were on it as well. In theory you should still be able to copy your saves to a different drive if it goes past its write limits, but this would depend on whether the Wii U fails transfers if it can't set the erased bits on the original files (which is probably the case to prevent locked-save duplication).
I've used an SD for the past few years without a problem, but I have standby mode disabled and I honestly don't use it a whole lot. I don't know if the extra writes are as bad during normal use, but probably. I'm just not too concerned about losing my saves so it's not a big deal to me either way.
tl;dr: HDDs are obviously preferred, but make a judgment call.