Pokémon from flashcarts are the same as Pokémon from legitimate cartridges. As long as you don't use cheats or anything you'll be fine.
I've personally never used a GBA flashcart, but I've read good things about the EZ Flash Omega, which is the newest one I'm aware of. A quick and not-at-all-thorough search of the shops I usually go to makes me thing the others are no longer made. An emulator might be the next best thing if the Omega doesn't suit you and if you can't find anything anywhere else.
This is something I've been wanting for years... And I'm sure many of you have been too. I finally made codes to modify what nature you get when taking the personality test. These natures correspond to the ones here.
These are for the North American version, and do not require that you press any buttons.
Lonely: 2237cbb7 000000FF
Docile: 2237cbad 000000FF
Quirky: 2237cbb8 000000FF
Brave: 2237cbae 000000FF
Calm: 2237cbb5 000000FF
Timid: 2237cbb2 000000FF
Jolly: 2237cbaf 000000FF
Relaxed: 2037cbb6 000000FF
Quiet: 2237cbb9 000000FF
Hardy: 2237cbac 000000FF
Rash: 2237cbba 000000FF
Bold: 2237cbbb 000000FF
Naive: 2237cbb1 000000FF
Impish: 2237cbb0 000000FF
Hasty: 2237cbb3 000000FF
Sassy: 2237cbb4 000000FF
Players that visit any Pokemon Center on their birthday month, starting from November 16, 2018, will be eligible to receive a Chansey serial code, provided they show the Switch and game software.