This is not about the output, but about the timing:
frames per second: 50 on PAL, 60 on NTSC
C64 cycles per line: 63 on PAL, 65 on NTSC
VIC-II total lines: 312 on PAL, 263 on NTSC
When you're coding effects that are close to the hardware, you can't just switch from one system to the other. E.g. almost all demos for the C64 run on PAL hardware only.