I have magnets on my hatches. They work great and I really like the look. My only electronics experience is with my digital camera and I have had no trouble with that (and would not expect any).
The one side effect that I had not read about had to do with sand. During my paddle on the Columbia I found that sand here has enough iron in it to be attracted to the magnets - creating a series of dots on my hatches and hatch lips.
Not a big deal, I just make sure the lip is wiped clean before sealing the hatch - something one should do anyway. The only other issue is that it turns out that 8 year old boys like to throw sand on my kayak and watch it stick on the hatches!
I covered my magnets with a thin (1/32") veneer of wood and then fiberglass on both the hatch and lip. The holding power drops off fairly quickly with increasing separation gap and I had to double up some of the magnets in the lip to get the holding force I wanted.

Steve