I will say that I was helping with a dyno session the other day and the first three timing lights we tried would not fire with the EFI system that the engine had. The engine builder happened to have 4 timing lights in the shop (all different brands). The fourth one worked which was a good thing since the dyno session would've been a bust if we couldn't set the timing. I've never known a guy to have 4 timing lights before (I only own 2 myself) but I'm glad he did since it saved our bacon.
I should add that we had to use a stand alone battery with the fourth timing light. That engine was touchy!