I've had no end of trouble with Yamaha motors (Giant Syncsport). Granted, it's only one bike--but five motors in a year? I'd be wary until some long-term reviews come in.
FWIW, an LBS local to me that sell e-bikes exclusively told me they no longer bring in bikes with Yamaha drives because of too many issues with them in the past. I didn't ask whether Yamaha was a pain to deal with or the drives were problematic.