I spoke with Eric Sovern at Surly Bikes about the widening of fat bike rear ends and he helped shed some light on the topic. Surly made the Pugsley as the worlds first production fat bike over 10 years ago and they used a 135mm rear hub and off set it 17.5mm so you could get a full range of gears. As the math works 17.5 x 2 + 135 = 170mm, they were eventually able to have 170mm hubs made and this became the standard.
170mm works quite well for an all around hub standard but it lacks a little if you want to run a 5" wide tire. They increased the hubs to 190 mm with the thought that you could always put a 4" tire on a 190mm but would struggle to put a 5" tire on a 170mm. This is why most companies have switched to 190mm. This is all so new that it is happening in real time with changes being made every year, hence why everything was 170mm and now is 190mm after just a couple years.
Eric said not to be afraid to run a 190mm fat bike as your 4 season ride as thats where the standard is seeming to stay, he just recommended that you double check that it has as close to regular trail bike geometry as possible. You could always run 4" tires in the summer and 5" in the winter. He suggested their Wednesday fat bike with an added Bluto from RockShox as your 170mm option.
I am going to continue to speak to a few industry people and try to ride a few more fat bikes before releasing an article on the issue, but because you asked a good question you got to know first!