But smartphones with stronger radiation and ultrafast processors and whatnot have been around for a relatively short time.
Here, I just randomly picked a popular phone from 2006, Morotola Razr, and Motorola Turbo Droid, from 2014:
Razr SAR rating:
Droid Turbo SAR rating:
SAR US 1.39 W/kg (head) 0.50 W/kg (body)
Just two points but I imagine more search would show the trend is that SAR is getting higher.
I assume there is a point where harm begins to show -- imagine you build a phone with SAR rating of 100W/kg and use it every day. Would the effect show in 100 years, 50 years, 10, 1, six months...? So the question is where that point is for the what seems like a very common 1.39W/kg.
And then there is the question how reliable SAR is as a measure of effect of radiation on the tissue. From the Wiki page: "SAR limits set by law don't consider that the human body is particularly sensitive to the power peaks or frequencies responsible for the microwave hearing effect. Frey reports that the microwave hearing effect occurs with average power density exposures of 400 w/cm2, well below SAR limits (as set by government regulations)"
I don't think it's unreasonable to say that not enough time has passed for the new generation of phones to rely on it as evidence of safety.