There's only 2 things that could make me return the device and I don't think either will be an issue:
1. Cell reception where I work. I have Verizon and my iPhone 6S Plus gets good reception here. If for whatever reason the S8 doesn't that would be a deal breaker.
2. Camera - specifically recording video. Most sample videos I've seen shows some jitters or frame wiggle when panning across wide areas. My iPhone 6S Plus is perfectly smooth in this regard. I'm sure the S8 takes better still pictures than the iPhone 6S Plus but I take a lot of videos and if I can't record smooth videos with the S8 that'd be a deal breaker also.