I would agree that the game becomes progressively difficult in direct proportion to population. After you hit one million, in my opinion, the difficulty hits a plateau. It remains a challenge to maintain resources for your city, but once you get the hang of it, it becomes 2nd nature. With that in mind, though, I do think the game is too easy to master.
Back to SC4, as others have mentioned, I'm still tinkering and learning after what seems like a decade of playing it. 1/2 the fun (and frustration) is filing through the hordes of user created bats/mods/models/nams/etc. Cities XL cannot compete with this because it's new, so hopefully that complaint gets treated with contempt. However, it would have been a serious nod to the community if MC had given more than passing thought to custom content.
Had that happened, and players were able to implement some type of their own models into the game, I bet we'd be hearing more about great looking new buildings and a little bit less about XL's shortcomings.
In the end, yeah, XL is way too easy when it works properly. I say that because XL's trade system isn't exactly sailing in calm waters. As it stands, it's really difficult to get anything done, but if it worked the way it should, it'd be easy.