I know almost nothing about Fitbit, but for the most part their image is positive. Could you elaborate on what's wrong with Fitbit devices?

I've been pretty happy with the devices, when they work. But they don't seem built to last. Of the 6 Fitbits I've had in the last 5 years (the original, 2 Fitbit Ultras, 3 Fitbit Zips), only one of them didn't fall apart or otherwise stop working .. and that one I lost. The Zip in particular seems to have an issue with the battery cover, where it wears out and becomes too loose and the device won't stay powered on.

Exactly. Build quality, reliability, etc. Very cheaply made in my experience.