Indeed, the dynamic AI made Forza 4 almost unplayable because it didn't adjust properly, it was just always too slow.
In general AI is one of the biggest problems racing games have had for the whole generation. Too slow (for example Forza series), too fast, too stupid (Shift), too inconsistent (F1 series), too consistent, too much rubberbanding, not enough rubberbanding. Has anyone got it right in a console title in the past five years? Give me Simbin-style % slider and I'm good to go but sadly that won't happen because, well, you just can't have that in a console game.
One of the reasons based on the demo and reviews of Forza Horizon is that the races are too short so if you set the AI to hard or insane and have to start on the back row, you never have a proper chance to catch up the cars starting on the front row because the race is over in a minute or two. Using ramming tactics on corners helps a lot but some people prefer to drive clean. Now this is obviously not limited to Forza Horizon, it is a problem with many racing games.
