Simon I have done a lot of camera calibrations on phones (it is necessary for doing any kind of computer vision/augmented reality) and they really don't distort that much, even at full wide setting. We aren't talking early 2000 camera phones anymore. For example iPhone/iPad cameras gave me a calibration that was pretty much dead on match to an ideal pinhole camera. Square pixels, distortion coefficients down in the noise. I am sure the physical camera isn't so great and Apple's firmware does some undistortion/pre-processing before delivering the image to the userland APIs (I am not talking about the camera app but images grabbed from the camera using my own code) but the user doesn't really care about that.
SLR lenses often distort much more than tiny smartphone lenses, especially barrel distortion at the wide end can be really pronounced. The smartphone lenses with their microscopic focal length don't really have where to pick that kind of distortion up at the distances the lens is usually being used (and is even able to focus). It is extremely difficult to build anything resembling a fisheye in such small form factor. That doesn't mean it is impossible to build a crappy lens but given the smartphone prices today the customers would lynch such company.
Of course there is no comparison with an SLR when it comes to sharpness, color aberrations and minimal aperture/low light capabilities but you really don't care about any of that when shooting pictures of PCBs for analysis. For this kind of job an SLR is an enormous overkill, even a crappy handheld phone shot is going to be usable just fine. The idea is to enable reverse engineering boards, not to win a photography contest! (and yes, I do own a Canon 80D with a few lenses too)
Also often when you are fixing boards you don't have an SLR at hand but almost everyone has a phone - then the whole point is moot.
Anyway, if you happen to have a phone that produces images so distorted that it would cause problems with the reverse engineering, it is pretty simple to calibrate the camera and remove the distortion in software after the fact. Definitely simpler than having to monkey around with a large camera, tripod and what not - and have to undistort the image anyway ...