The SF-24D is actually a pretty good flash.
I have to love that M8 owners are so often fixated on using the wrong (i.e. Canon) flash with the M8. It's wrong because Nikon's flash system is 100% more compatible with the M8 than Canon's is.
Why?
1. The hot shoe pins on a Nikon flash are in the right place. That's a good start.
2. The A (Auto) mode seems to be missing from most Canon flashes. Unless you are a serious guide number fiend, a flash with just TTL and M modes is clumsy. Nikon loves A mode, and most of its flashes have it.
3. The Nikon flashes have a pilot light pin that correctly activates the sync indicator (and if appropriate, sync speed) in the M8. The Canon flashes I have tried do not.
3a. The flash pilot indicator also tells the camera that it can (in Auto ISO mode) to drop the ISO to 160.
4. The flip-locking pins on newer Nikon flashes seem to function in the M8's hot shoe.
5. A lot of Canon flashes have very loud recycling noises.
A Konica HX-18W flash (from the Hexar RF) is also a workable flash for the M8 (although it only has two auto modes - i.e., apertures).