The problem is that the game sets the difficulty at the start, and only really ups the ante once. In OldCom, the aliens are strong from the outset, and gradually get stronger - but through all the time you spend fighting, you are playing catch-up on sheer power. Your best tech is the alien tech, and you have parity with the aliens on the ground - a shot of Heavy Plasma can OHKO your best soldier if he is unlucky, and Heavy Plasma is standard alien gear by then. In the air, you need massive investment to achieve that parity - Fusion weapons and Firestorms are expensive to field, and the Aliens can mobilize in far greater numbers than you can respond to - and until then you are SOL if you meet a Battleship.
In NewCom, the combat system is such that a hit of even a Mechtoid's or Sectopod's plasma cannon, even a lucky one, will never kill a fully kitted out Colonel outright, whereas a shot of a Laser Rifle, sans Damage Roulette, is a guaranteed kill on Sectoids and Thin Men on Normal. And Sectoids stop being a danger at all once you have Titan armor, as well as Thin Men to a lesser extent. Even Chryssalids stop being a problem once you're up to Plasma, since you have abilities that straight-up counter them - CQS plus Alloy Cannon. Simply put, in OldCom any alien you saw was a danger, all the time, whereas in NewCom you have "tiers" of aliens and equipment, and can ROFLstomp aliens on a lower tier than you. Same with Geoscape - Abductions are a simple event, UFOs appear one at a time, Battleships only appear in very specific circumstances, etc - only one Firestorm with the best cannon is ever required per continent, and the interception is an arcade game with "powerups" instead of anything like a risk-filled engagement.
With such definite mechanics and counters, of course the game stops being challenging. It has a definite victory clause, and you end up always more powerful than the aliens instead of fighting a losing battle and needing to Ender's Game your way out of the invasion.