I find they help physically. After a few hours of driving I feel less tired and need to rest arms/legs physically.

The mental toll is nearly unchanged.

So when you stop I feel it’s easier to walk around and shop or whatever since I haven’t been applying as much force constantly.

But you still need that mental brake.

I find adaptive cruise control definitely helps with the mental fatigue as well, because I can concentrate on looking for hazards far ahead instead of having to pay attention to keeping distance to the car ahead. But yeah, I never drive more than a few hours without taking a break anyway.