* They run an ancient version of android (Android 6 or Android 5, I don't remember)
* This ancient version of android runs a kernel that is susceptible to multiple vulnerabilities (like DirtyCOW)[0], and the developers have disabled selinux
* Kernel sources are not released, in violation of the GPL and the last time I checked, it seemed like there was evidence of ripped off applications/IP theft in bundled but hidden applications.
* The bootloader is locked
* Access to the wacom digitizer is only possible through a proprietary blob, which has to be incorporated into an application that wishes to use it. Therefore, the stylus support has a tremendous lag for any application that is not an onyx-made application.
* Handwriting recognition requires connection to their cloud services
Their forums are down right now, but people have been begging them to at least comply with the GPL and release kernel sources for years now [1,2]. The last time I checked, there were 3 separate threads about this on their front page.
It's a really tragic situation for such nice hardware. I can't think of any reasons why the company wouldn't want to open the source unless they were hiding something or were afraid of getting ripped off by other OEMs. The only thing that the onyx devices had over the ReMarkable is storage, but seeing as how the RM can be modded to accept an SD card[3] and has first-class linux support, it's the better choice IMO.
[0]: https://www.mobileread.com/forums/showpost.php?s=42e53ed5d56...
[1]: https://web.archive.org/web/20190122063838/http://bbs.onyx-i...
[2]: http://bbs.onyx-international.com/t/install-linux-or-alterna...