I view it as similar to an EMP. The wave disables most technology, but, well, with a bit of work you can simply switch an engine back on again. Lightbulbs might do little more than flicker. Even a biotic implant won't kill you if it simply stops working.
However, if a synthetic mind is switched off for a short amount of time, you can't just switch it back on. A bit like how the brain doesn't work any more if it stops getting oxygen. Synthetic minds require quantum computing in the MEU, and quantum processes are notoriously unstable (and this is in real world physics - seriously, just look at anything quantum and you'll disturb it). So I imagine that switching the power off kills the mind.
As for the organic parts of the Reapers - presumably they rely on the synthetic parts being switched on to carry on surviving? (This brings up scary implications regarding people on life-support throughout the galaxy.)
There. Some speculation and headcanon, but it's a good enough answer for me.
In my opinion, Synthesis is a whole other ball-park of unreal.