* Jolla has always used (and continues to use) outdated hardware, Ubuntu Phone OS shipped with or ran on reasonably performant hardware. Only after several years is Jolla now discussing the possibility of porting the OS to a somewhat modern device (Sony Xperia)
* The Sailfish SDK relies on VirtualBox for the emulator and for native compilation, a drain on resources and memory, and an indication that the development team was not well versed in creating cross compilation ecosystems or tools. Ubuntu Phone SDK was limited to Ubuntu Linux, but ran natively (Sailfish SDK required VirtualBox even on Linux)
* For years, Jolla withheld from or misled the public on the state of internal affairs (Canceled tablet, no refunds for crowdfunders, CEO and key employees quitting, lack of funding, relying on volunteers for key areas of development)
* Sailfish OS has always lacked basic security features for applications (QT Quick embedded in plain text) or for the phone itself (encryption, permissions). Sailfish's security problems may be just as problematic as Tizen's
* Sailfish OS was never true open source, even less so than Android, although it has always been advertised as such.
* Very poor/non-existent developer relations - A few developers attempted to make games for Jolla, but encountered serious problems that the developers weren't interested in fixing (i.e. SDL2 on Jolla not supporting landscape mode, preventing games from being submitted), even worse, the developer for the SDL2 Wayland port flat-out refused to fix the problem or provide the source code for other people to fix the issue. Such problems or resistance from the developers were never encountered on Ubuntu Phone OS.