Quoted from swampfire:CLAIM: INCs can cause playfield plastics to warp
While not an insert lamp, I've seen an incandescent flasher, lock on and melt a plastic.
As far as seizures, it is not necessarily brightness that can cause it, but that in combination with the pattern/frequency. I suspect the old flashers would have the same issue, but I do think they use flasher differently or more extensively now which maybe can increase risk. However, As far as them posting a warning, could be nothing more than Stern's lawyers/insurance company wanting to reduce liability.