Can one trust OS and apps from Onyx: app store, modified Android, Onyx Cloud

Onyx Boox is a brand of e-book reader produced by Onyx International Inc, based in China. They have e-book readers based on Android OS. They have features that can violate user privacy or other accounts security:

  1. App store with optimized for e-book apps from other App stores: kindle, office, evernote and etc
  2. Account manager: Dropbox, Evernote and etc
  3. Option to enable Google play and other Google services (like Calendar)
  4. Onyx Cloud (sync personal notes and etc)

So the question is: are there any sings of backdoors or vulnerabilities known about their modified apps or OS itself or other stuff that can lead to user data leaks (like privacy leaks or leaks of sensitive information: like passwords or other data)?