Well, there are a few things I don't like about NA in AI hands:
1. Techs miserably
2. Loves espionage and poisoning water supplies, even if he's otherwise at a good disposition
3. Builds a ton of units
4. Very costly to take over early in the game
5. By far the worst thing about SB: He spams demands constantly. Popups galore!
Some good things though:
1. Doesn't declare @ pleased
2. Warmongers HATE him and will declare on him due to peace weight variance
3. Because of #3 and #4 above, they rarely succeed.
Overall he's an AI I don't mind having in the game because he's rarely a threat to win the game and he disrupts the difficulty associated with some of the other AIs. Cannons and later take care of him and all PRO civs nicely.
In human hands, the started techs and PHI are good enough to make NA decent. Axe rushing is a pain, but that is compensated by pure barb immunity for some heavy expansion...and it's easy to hold those cities even from the AI.