Challenge level should be about a smart computer opponent, not a cheating computer opponent. This game is rigged.
A board game that involves dice is about chance, but chance is completely removed from the equation when it comes to a video game because of programming. The programming for this game is all about giving you the shaft while giving the computer all the good stuff.
When you've played, in between your turns, how many times have you lost two or more cards? How many times has a computer opponent played a soldier to take a card from you and then roll a seven to take another card from you? And how many times has the next computer opponent followed by doing the exact same thing?
How many times have you had resources on sixes or eights that never got rolled, yet the computer was raking in the resources on twos and threes? Then when the robber was on your six or eight, suddenly those numbers were being rolled like there was no tomorrow?
On one-player mode, you're not playing against computer opponents -- they're playing together against you.
What this means is that multiplayer is the only way to go because the computer won't screw you over and over again. But if you have people to play against, you'd probably be better off playing the actual board game.
On a side note, I find it interesting how the reviews with the lowest ratings were given a lot of thumbs down. It's as if those knee-jerk thumbs down reactions are from a bunch of Brian Kuhs and Catan is the Billy Mitchell they worship unconditionally.