I've can't comment on the phone method as have never tried it, but my understanding is that phones rely entirely on their internal gyros for position.
As a result you only get 2 or 3 degrees of freedom instead of the 6 you get out of a rift/vive.
That should be enough to be fair bit I've no idea what sort of angular resolution a phone can achieve, which might prove to be immersion breaking (Rift/Vive will position you to sub mm accuracy within the play area).
Another important one is to make sure that any phone option you use allows you to focus to infinity - otherwise you'll have serious eye strain after even sort usage, a full race would probably be painful.
Rift/Vive trick you into seeing in 3D, I don't know if phones apps can do this? And it's the depth perception from the 3D which is the biggest benifit in VR when racing . The whole being
the car having to look at mirrors and being able to get out while parked thing is just icing.
