Ugh, that's not good. I've seen a number of posters who stated they did NOT have the rainbow effect while wearing polarized sunglasses. I have 3M Crystalline 70% on two of my vehicles and they certainly do have a rainbow effect as well as a bluish tint.
For those that want to really evaluate the heat rejection performance of window film, look at the Total Solar Heat Rejection value (TSER).
3M Crystalline TSER at 70% = 50%
XPEL Prime XR Plus at 70% = 49%
3M Crystalline TSER at 40% = 60%
XPEL Prime XR Plus TSER at 35% = 59%
As you can see, moving to a much darker tint doesn't necessarily give you that much more heat rejection. The difference of TSER going from 70% to 35-40% tint is about 10%. 10% is not insignificant, but IMO it's not a game changer.
For those of us in California, tint is technically illegal. Since OEM glass already has some tint (dye) in it, a 70% tint is additive and will darken the light transmissibility to a level below the legal limit. However if your rear windows are darker and the front at 70%, the hope is most cops will leave you alone.
Also as someone who had really dark tint on my rear windows in the past, dark tint really makes it much more difficult to see out at night.
Therefore I'm likely to go with a 70% tint all around including the windshield this go round. The existing oem dyed rear side and hatch windows will make the 70% tint appear darker in the rear. That will help it look like the front is near clear and will draw less attention. Also the smallish 10% increase in TSER going to a much darker tint, won't make that much of a difference.
The problem is I need to find a tint that won't cause rainbows when I wear polarized sunglasses.
