You can avoid reaction fire with smoke & melee combat.
I know that, but you and the others are missing the point. My problem isn't with reaction fire or how to deal with it, the problem is that the behavior of some units are grossly incongruent with the rest, which leads to lack of immersion, paranoia about whether or not you can only take one step before the unit one-shots you, and missions that become way longer and much more tedious because of it.
The worst example was in a base defense mission. I had made the layout so the enemies had to progress like this: access lift > hangar > hangar > hangar > chokepoint. There was more to it inside my base, but it was a really effective layout and normally that's what i would go with. After i had cleared the initial wave however, no more enemies came. I figured they were on their way because they started further back, but no. I started skipping 20 turns in a row, just to see if they would come to me, but no. Because of this bug/feature, 6-7 enemies were just hanging out in the access lift, which meant that all that base defense preparation i had made was now useless. I had move so many troops all the way down there, and then breach the access lift against full-TU-enemies. This made the mission much longer and much more dangerous than it should have been, and the thought of having to go through all that again at some point, made me start a new game...this time with the access lift first, even though it was a much less efficient layout.
That's why it's not a question of dealing with reaction fire, but much worse than that. Given that i don't remember seeing this happen in other mods, i still think it's a bug, but if anyone thinks it should be a feature, they clearly haven't been on the receiving end of it.