Drifting with ASM Understeer on is like riding a bike with training wheels ~ Boundary Layer
Seriously, it is. The driving aids are there to allow the game to look after car balance and excess wheelspin for you. With them on your only job is to steer and accelerate, no consideration needs to be given to weight transfer and momentum. Using any driving aids in your drift setup greatly takes away from the overall gaming and drifting experience in my opinion.
I have always viewed their use as allowing the game to do my work for me, they have the ability to take a sim game with a great physics model and dumb it down to the point of being an arcade racer. I guarantee none of the top drifters on this forum use any of the aids, they have learned exceptional car control without them.
If youre looking for a greater appreciation or understanding of the game physics, car setup and control, and/or weight transfer then their use will only impede your progress. TURN OFF THE AIDS, you will become a better drifter for doing so. If you choose to leave them on, well thats your choice, but know that you would be building skill and learning technique faster without them.