Here are my 2 little pennies
I don't speak on behalf of SMB, but I'm pretty sure the main reason they use the Atlas is because its the only brand new case they can get. The 271 that came behind the 5R110 transmission hasn't been available from Ford for quite some time. So, why not use a new t-case to go with the new Dynatrac axle? The ability to only engage the front axle is cool, but is it really needed on these vans? In my opinion, no. The variety of ratios is nice. We're doing our 1st Atlas install soon (as soon as we get it, 6-8 week build time)
The 271 is a great t-case, and it has flanges front & rear. So if you had a rear ujoint or driveshaft failure you can pull the shaft and go home on the front. Even then, its not a great idea to be running highway speeds on that front axle, emergency only. I've seen guys do this, pinion bearings on a HP axle don't like it much!
I've used 271's in every conversion that I've done. The only time I've ever seen one fail is when the front driveshaft bottomed out and broke it off (this didn't happen on a van) Sourcing them in manual shift form is getting difficult, so I'm pretty sure we'll be doing more Atlas installs in the near future.