I test rode those automatic 3 speed bikes from Trek. Here's what I found.

The transmission was constantly thinking about shifting but couldn't execute perfectly all the time. The Nexus 3 speed internal hub was not meant to be shifted under load because you will hear a loud crack when the drive get stuck between gears!! While this is fairly easy to avoid when you have control, it's not the case when the bike is doing the shifting for you!

I would constantly cause the transmission to "Crack" as the transmission struggled between gears. I'm sure Trek had a lot of the bikes coming back for warranty. Even Shimano stopped making the group set.