This has probably been covered (I haven't read through the entire thread), but my biggest complaint about XCX was the rate of progression in terms of Skell availability vs Continents accessed.
Basically, I feel as though I had only really had a chance to explore Primordia, Noctilum and 20%ish of Oblivia before getting access to a flying Skell. Basically once I could fly, I had zero reason to run, so I just flew around, unlocking Data Probes so I could quick travel.
I hadn't even seen Cauldros at this stage (some 45hours in). So I just flew around. I remember thinking, 'man this is an awesome castle overlooking a ravine, it would have been epic climbing these stairs' but I didn't get this opportunity, because i had zero incentive to.
Once the Skells come in, I had faster and easier transport, and more firepower for combat, rendering on-foot gameplay pretty much useless.
This is on me, I get that. The option to be on foot was there (I chose to use the Skell), but the balance of the game, and the rewards for non-Skell were severely lacking.
I really think that flying Skells should have been a post-game unlock, with areas only accessible via Skells being your End Game zones.