Since rakan can E to his adc, if you're in range of his adc you're basically in range of rakan. And actually if he's behind his adc he gets a tiny bit more range from his e putting him ahead of the adc I think. Main point is you don't want to get cced in front of the enemy adc, and rakan can just E to his adc and W you.

When is it ok to get cced? If the enemy adc can't follow. So you can sort of walk up to rakan himself if his adc isn't close enough to follow a rakan W on you.