As far as I can tell, the emulator widescreen hacks are hit and miss, while specific widescreen cheats were tested for a specific game, giving you better results overall, for instance, Gran Turismo 2 will display garbage in the left and right sides using the emulator hack. Using specific cheats you can not only make it look OK, but also run it in 60FPS, change the meter to KM/h, etc.
Note the HUD is streched here, but usually, even HUD elements are kept in their original aspect ratio if the cheat accounts for that. Personally, I’d rather use a cheat like this, with increased internal resolution, 60 FPS, L2 and R2 analog throttling/breaking.
So, basically, stick to specific cheats rather than relying on the emulator hacks. Unless the emulator devs actually adds these cheats to the emu core itself or provide those game specific cheats, like PCSX2 does.