Rainbow isn't really the right term. It's more of a sparkling effect. Apple actually uses the term "sparkle" for this characteristic in their patent for the display treatment (see para 0073). They also mention that different diffractive layers can be used to minimize the effect, so it is possible that the issue is worse on some devices than others.
Well whatever it is, my MBP screen looks perfectly fine. It's like a matte display, but not with the colors all washed out like the matte displays of yore. No visible artifacts of any kind that I can see.
