Happily this isn't always the case with CSM players. I had some very lucky rolls in a game last week, 2k of my Nids vs his CSM.
Just to be clear, I use a shooty HT, 2x6 shooty Warriors (one of them in a spore), 1x10 Genestealers with scythes and toxin, Trygon Prime, T-fex and 2xshooty fex. Small, not swarmy, effective vs GEq, not so much vs MEq.
I did not lose a single unit and reduced his whole army to a unit 6 plague marines. I was also holding all but 1 of the objectives. I had some very lucky rolls.
The CSM player was understandably upset, well probably more shaken and stunned, but put a brave face on it (only moaning about the luck a few times). We were both using the same dice and there were no rule or unit disputes.
He was a pretty good sport considering the total destruction he was dealt, while not killing any of my units.
All that is important is known.