Are flares effective countermeasures?

I’m scratching my head about whether flares actually work against all types of missiles. Like aren’t they only effective against IR? Other (radar) weapons should respond to chaff.

I am still a newcomer to the game, but flares just don’t seem to be enough to disrupt missiles from 5th generation fighter s who just hang back and lob them at you from afar.

Is there any way to make matches guns only or limited to 3rd or 4th gen fighters?