Not everyone shares this view, but IMHO, primarily 2 things:

1. They run an outdated fork of Android (6) that is no longer being patched for security issues by Google, and

2. They are incapable of efficiently running anything but a small subset of current apps in the Android store, which effectively renders them useless for one of the main purposes people buy them (a "real" e-ink Android device).

Read the long threads that go on endlessly about does app x work, does app y work, and you'll see what I mean. On a real Android device, you don't need to have this discussion about the vast majority of apps. Or at least, the number of instances like that is far smaller.