The UI wasn't doing multitasking correctly. The [X] button was [_] button for no good reason, and users tended to leave bunch of apps burning battery not knowing how to close them(e.g. through a third party quick task switcher-killer).
The OS kernel wasn't so stable relative to anything else at that time including embedded Linux, had 32MB/process hard RAM limit that ruled out WebKit, and also the scheduler was badly designed that the UI frequently stuttered or locked up. The UI also did not utilize GPU for general UI rendering(common at the time; made an obsolete idea by Apple), tasking CPU and its bus for everything.
The device I had came with a stylus shaped like a handstrap charm that doubled as the reset button pick. Isn't that inexcusable by modern standards? Everything back then did have a recessed reset button hole, but not everything came with a carry-on tool to poke it. The WM device I had did. Which I didn't use as lifting and reseating the battery was faster.