I heard someone tested it and it does and I'd agree. Hansel managed to master every quest as a hansel build, yet I'm lc with only one guild and i can only beat the 60th/70 quest 1/5 times approx. There's no way that unless it counted he'd be able to beat the last quest 100 times unless he was super super lucky or took forever trying
I am absolutely convinced allies matter as my quest success rate drops significantly when my ally stats are down. I have managed a better success rate with weaker attack stats so long as my ally stats are up. I have seen this argued both ways but without a definitive answer from the devs I am inclined to say yes, ally stats impact your success rate.