the + aligns on the math axis so one can argue if the type-one variant is ok ... so we would need a smaller (less height) + then which would look visually weird
Plain TeX doesn't align the plus/minus to the math axis. Rather, the minus is aligned to the baseline, and the horizontal stroke of the plus is above the baseline. I agree that it looks a bit strange with the horizontal stroke of the plus sign not aligned with the horizontal stroke of the plus/minus. Bt it looks even more jarring to have the minus below the baseline, because the baseline is such a strong visual line. I don't think I've ever the minus below the baseline in a math book, and it looks very wrong to my eye. Pagella looks worse than the others, because it has a vertical gap between the minus and the plus sign, so the depth below the baseline is even larger.
(all can be achieved ... but the idea of lm/gyre fonts is to provide an acceptable consistent set ... at some point we can think of variants - or maybe the fonts have a variant already)
I realize that it's not a ConTeXt problem, but is there a ConTeXt workaround? Or better to go to the TeX guts and compute the depth of the box, then use \raise{}? -Sanjoy