In my personal opinion, the problem with the XCF endgame arc is that it typically occurs so late in the game and/or after waiting so that you have already maximised all the relevant agents, researched everything, etc. And looking at SL streams, psionics is also overpowered in the late game (you can MC all the ethereals and alien sectopods as well - the situation might be different if it was very difficult or impossible to MC ethereals). Especially MC enemies which have good psiVision takes much of the surprise and excitement out of the later game.
So the most important and interesting parts of the game are over once you reach the point of going to the moon/orbit/mars and what's left is essentially just waiting for the RNG and sequential events to follow each other (and skipping all the rest or doing essentially useless missions).
Game design wise this could be improved by making the end-game arc start earlier and maybe not being so linear. For example, requiring an alien commander rather than ethereal commander to get to at least some parts of the end-game arc, and making avenger less dependent on RNG.
Also from the story perspective, I'm not "fully convinced" that the idea of learning about MIB lunar base can be squeezed out of MIBs on earth (and you already can do other missions on moon, like UAC moon base), but you can only infiltrate it once you obtain the keys to alien-MIB communications from ethereal commander (shouldn't the MIB commander be able to tell you the same things). While this is framed in the story as "posing as the other side", going to the MIB Lunar Base mission does not require any technology where your craft would actually masquarade as an alien craft in a manner that would make sense. (In order for it to make sense, the prerequisite should actually be obtaining and learning to use a live space fight capable alien craft and riding THAT to the MIB moon base, a bit similar to how in TWoTS mod you need a third-party alien infiltration craft to get to T'Leth)
But because this part of the mod is already well-established, I don't hold my breath for significant changes. Like in my previous campaigns, personally I just very likely get bored (once again) and stop playing the campaign at the latest when I get to this waiting and/or figuring out the right way to obtain the right ethereals phase.
