I think the rumor that it's fatal probably originated from the idea that DXM potentiates opiates. The best designed study I've read found that DXM did NOT do a thing to potentiate morphine at 1:1 ratios. That doesn't mean it doesn't work to potentiate other opiates, or that it doesn't work at higher doses, or that it just doesn't plain mix well with opiates, but morphine was the one that was thought to be best potentiated by it. They made a brand called Morphidex using the combo because even the pharma community believed it.
http://www.painjournalonline.com/article/S0304-3959(05)00108-9/abstract
I quite like the mix, but so far as I can tell they only MIX well (because DXM is has some opiate like qualities for me). To me, 10 mg morphine plus 10 mg DXM definitely doesn't feel like 20 mg of morphine the way the older findings circa 2000 implied it should.