UFO: Aftermath is built around some interesting ideas. The whole mating of giant organisms, creation of monsters out of human bodies, pilot rescue missions, "see every monster" missions, aliens stranded on Earth and joining my forces to fight the traitors... However, it was buggy and I find the engine inferior to the original UFO:
the operatives cannot enter buildings,
there is only one level,
the terrain is indestructible,
the ability to zoom in turned out to be useless — once you've seen each monster, there's nothing to look at closely.
What broke my patience, was the inability to research plasma weapons — looked like a bug to me. After that I dialed the difficulty to the lowest, played until the end with invulnerability cheat codes and didn't bother replaying the game or trying sequels. Turns out, I missed a few things.
I tried UFO: AI about 10 years ago. The AI was nonexistent, models were ugly, purchasing weapons required a lot of micromanagement, and I decided to wait a few years before trying it again. I haven't played it since then.
Xenonauts so far is a nice game. I loved the idea of detecting UFOs by "sightings" and "strafes", but it wasn't used much, at least on the lowest difficulty. The movement mechanics takes time to get used to, but is more realistic. So far the most unpleasant part was waiting several months without any UFOs. I fired scientists to save money, but did it too late and lost more from re-hiring. Then I accidentally wiped the disk and haven't restored the backup yet. Another problem is that there seems to be no technical support anymore — there are multiple crash reports on their site, but no answers; usually, I had to start over with saves from a few game days earlier.
I tried playing the 2012 XCOM game when it was released, got as far as the first Cryssalid, but Steam version kept crashing at certain points if the network was connected. This bug has probably been fixed, but I haven't revisited the series. What I hated about the game, was inability to build more bases and inability to send several teams to several simultaneous missions (did DLCs fix that too?).