I think the latest firmware update made the M8's SDHC compliant. Before that it was a 4GB max. With SDHC I think the max is now 32GB. Although I would never use a card that large personally. When I first got my M8 I used 1GB cards. And had about 5 of them. Then they stopped making them and I started getting more work and switched to 2GB cards that hold about 186 DNG images per card. When I get my M9 I plan on grabbing 5 4GB SD cards.
The reason I keep my capacity low is so that if a card becomes corrupted, all my eggs aren't in one basket. And cards will get corrupted. It's a given. It's also like having to change out rolls of film. Only being able to fit so many images on one card. It reminds you to pace yourself instead of banging out at 3 frames per second. I just make sure I have extra cards with me at all times. SD cards are pretty small and thin. Not a problem to keep a spare in a back pocket.