continuously increasing alien activity once you have played for a certain time
I thought about this specific point. If UFOs become extremely numerous in the end-game :
+ It makes good use of the Geoscape interface, with many dots buzzing around,
- But the notifications become very annoying (you probably have full coverage of main lands, with decoders which have 100% success rate)
+ The player gets progressive feedback that he's outnumbered, as he sees his interceptors spend less and less time idling, and soldiers always on duty
+ Once the players resources are fully mobilized, there's a real incentive of choosing specific missions : Protect important countries from being lost, ignore antarctica settlement(s), keep an eye for specific alien you need to capture. It's much more active than systematically answering every call.
- Much before getting overwhelmed in the game (limited by ship refilling), it's the player who will feel he's playing non-critical battles non-stop : A game month would take forever, because the player needs to play out 1 battle by game day, which all take at least 15 real-time minutes. An autofight system would solve this : manage the easy battles, play out tactically the critical ones.