SP was popular because it was what the Advance should have been to start. I got an Advance at release and the screen was really nice when I could strain my eyes enough to see anything.
My other favorite Nintendo “WTF” is anything to do with the N64 controller, look it up a teardown if you’re bored
My favourite WTF regarding the N64 controller was that it apparently re-zeroed the center point on startup but didn't monitor the range of readings in any way. So in some games you could speed hack by holding the joystick downwards/backwards on startup and and generate faster-than-100% run speeds.
Unfortunately not, it was just something I remember hearing/reading about back when N64s were somewhat relevant... I've had a look around but I can't find anything to back it up so maybe it was just an urban legend.
I think GP is referring to the unlit, relatively low-contract LCD screen that was only readable in bright light conditions.
The SP was better, but only somewhat: it had a button for front-lit lighting so the image quality was still pretty mediocre. I presume to save battery life, since a back-lit would have lead to much more light loss.
A couple years after the GBA SP (AGS-001) a new revision of it, the AGS-101, came to market.
That revision used a backlit screen and the toggle button allowed you to switch between a dim and bright backlight mode.
If my memory serves right games on that revision looked like they would on the original Nintendo DS.
Obviously battery life took a significant hit compared to playing on the AGS-001 with the frontlight disabled.
There was also the backlit Gameboy Light, which was only released in Japan. It was essentially a Gameboy Pocket with a electroluminescent back-light. It glowed green like those old Indiglo wrist watches.
I've got one, but sadly the LCD has a bunch of dead lines. Apparently, that can be repaired pretty easily.
My other favorite Nintendo “WTF” is anything to do with the N64 controller, look it up a teardown if you’re bored