There are several things to consider here
kitchen sink design... while it may look like working in proof of concept scenarios, it is hard to extend its functionality without breaking something
originally it was meant to be fast vector based UI framework, and then it got a whole load of styling and pixel perfect baggage - combine that with 1. result is bugs and slow performance
proof of concept library with limited set of widgets is easier to put together, to cover all that is needed to make usable cross-platform apps, FMX had to be extended with many additional widgets, behaviors, functionality - combine that to 1 and 2 - again bugs...
different memory models on Windows and mobile platforms made it harder to debug various issues and aspects - more bugs
FMX significantly improved over the years, but its original kitchen sink design is still holding it back.
FMX on Linux is good (great) move considering the developer's needs. I just wonder how many kitchen sinks Embarcadero bought this time around.