X-Com randomness is bad because a bad roll might screw you, and a good one is not satisfying.
Compares to a roguelike.. You have ton of random stuff adding up: you roll for Initiative, then for hit chance, then for enemy dodge, then critical hit, then damage. And the odd of have the worst outcome, or the best one, are pretty slim. It's satisfying when everything goes your way (init / hit /Critical / max damage), it's not so bad when everything goes bad because you hardly can be 1-hit killed.
If I remember correctly the first X-Com (from the 90's in DOS) was a bit more like a RL (you had more crew member, each with more hitpoint, and the aliens did not get the weird first move when spotted, those were alleviating part of the randomness).. It was still infuriating at time.
Giving more hit point to your soldiers and to alien would be a great way to make the randomness more manageable. But like the 90's version (or a RL) it will become very tedious when you exchange fire for a few round with almost no effect.
Another frustrating bit is the fake randomess. In my experience it happen in X-Com. And developer explained how they did it in Civ5: Player expected to win almost every time when shown a 80% win chance. And win half the time with a 20% chance. So they developed a system to mimic those expectation. In the end, the percentage does not make any mathematical sense. And when fight after fight, you experience a 99% success rate on a 80% shown rate, it become more frustrating the 1 time when you miss.
Tricking player brain, even for their own enjoyment, always backfire in my opinion.
Playing "Tales of Maj Eyal" (800+ hours so far). A Very pretty , rich and enjoyable roguelike... You can be almost 1-hitted but you probably had it coming when it happen.