Once you get used to it it's much easier to hold a smooth driving line than with a stick. You just basically hold it, like you would with a wheel, rather than relying on often many (depending on finger dexterity) small, sometimes imprecise movements of your thumb.
The only place where motion control steering falls a bit short in my experience is when you find yourself in situations where you need to make a series of quick line adjustments, but I feel that's more of an issue in a very fast arcade/simcade racer like DriveClub (and even there motion controls work very well - I can't handle it in WipEout though). In those situations the thumbstick can even have an advantage over a steering wheel.
I'm currently playing Project Cars 2 with motion controls while I wait for GTS. It works really well there too - hell, I'm better and less mistake prone with motion controls than with my FFB wheel (although obviously nothing beats the fun and realism of using a wheel, I just can't be bothered to set it up very often anymore).