True, but those issues are less critical as applied to the remote ground conditions. Any disturbances in the area of influence of the electrode being tested will just be correctly detected as affecting the real electrode to remote, infinite, earth resistance instead of messing up a very sensitive measurement that assumes uniform conditions over a wide area.
It is true that if part of the reference electrode system is within the sphere of influence of the electrode being tested the clamp method will read too low a resistance.
The three point method, by its nature, will be harder to apply for a distributed electrode like a UFER or a ring than for a simple rod. The clamp method is less sensitive to the geometry of the tested electrode. (IMHO)