I've done fairly extensive testing because multiple people have complained about the dice rolls, as always seems to happen for any video game with any kind of random element. Some games do fudge either in the rolls or the displayed percentages. I think xcom shows a lower hit % than your actual chance, which reduces player complaints for missing multiple 90%shots. In my testing both in normal gameplay and with stacked ac so that enemies require 20's to hit, dice results do not support any form of systematic bias. Either towards rolling 20's or 1's or hitting, or whatever other pattern you might complain about. It is clear from hours of meticulously recording every roll in game that the in game results are at least as fair as the cheap plastic dice i use for dnd irl, and likely better. With real dice I managed to go an entire 4 hour game session once with no roll higher than a 5 on a d20. Sometimes the dice hate you. The only thing I hit that night was a zombie, and that by the skin of my teeth. Nothing special wrong with the dice in question, I've used it with normal results on other days, just a 4 hour long string of improbably bad luck. If you are losing fights to bad dice rolls, you are letting your enemies roll too many dice at too good of odds. About the worst thing I can think swinging a fight luck wise is a critical hit with inflict wounds or similar that needed a 20 to hit, and that shouldn't be fatal unless you are playing with enemy damage above 100%. For reference from my tracking dice rolls, in my normal play, the party rolls about 4-5x as many dice as enemies. I have advantage more often, surprise more often, and do everything I can to deny opportunities to enemies with control and positioning. It took a long time to get enough rolls from enemies in normal gameplay to say they aren't cheating with confidence, because optomal gameplay is to minimize how many times enemies even get to try to hit you.