In Career mode, they have "dynamic a.i." and first year race, all the a.i. drive worst than me which is saying a lot. If you switch to the grid career path (ie pick and choose your race instead of game recommended) you can choose the a.i. and at highest setting they are pretty good.
I think they want to have the game feeling of progression where race get harder as you goes but for some of us who is already expert driving it can be off putting. They should let you select career difficulty level and if you select hardest mode, ai should drive at expert level and top car should be tune to be slightly better than yours or similar.
Anyway, still enjoy the game a log and rival mode is really awesome.