So I played the original X-COM games and was really loving Xenonauts, until the point a few months in when the squadrons of 3 heavy fighters are introduced, as well as the medium craft with heavy fighter escorts and the heavy craft.
At this point, in my opinion, it's impossibly hard to take down any alien craft, and because these are the only types of UFOs encountered, that I found (after 10-12 encounters all comprising the types above), I was unable to actually gather any resources to continue the game.
I'm very concerned that the game hits such an overwhelming difficulty spike where you can go from regularly taking out UFOs and engaging them on the ground, to being unable to take down any UFO whatsoever, seemingly immediately.
Even a lone medium landing ship can take all 4 of the MiG's torpedoes, all of the 2 Condor's missiles, and a bunch of gatling laser fire, and easily take down all of my aircraft. I had not yet been able to research the plasma aircraft guns, so this was the best tech that I had available.
Unfortunately, at this point the game stopped being fun, and became a major frustration, to the point where I no longer want to continue that game, because I feel that I'm in an inescapable situation that the game has put on me without warning. If I had done something wrong, such as sent too few aircraft or equipped them poorly, then I would be more understanding of this failure state, but to my knowledge there's absolutely nothing I could have done better to prepare for this type of combat.
Having read the forums, I understand that a better way is to focus on manufacturing earlier on, and focus on creating a Corsair, but I think that it's currently too easy to ruin an entire game, because one should be using forum posts to enhance one's play style, not to overcome unbeatable situations.
A better situation would be to introduce the harder craft more gradually. A squadron of two heavy fighters, or a heavy and a light, or perhaps the medium ships should be unescorted more frequently, or even throw in a good measure of the smaller UFOs, even later on, so that players who can't or won't research and build quickly can still have a chance. Another option would be to separate out the difficulty sliders for air and ground combat, so that players struggling with one can make this part of the game easier (even midway through) rather than feel utterly beaten and give up.
That's not to take anything away from the game or the developers at all. It's excellent fun and obviously the product of a tremendous amount of hard work; I will no doubt start a fresh game to continue enjoying it, but feel that I shouldn't have needed to.