Han's crew give Sean a lot of crap for being rubbish at drifting, but, Sean is used to "Street" racing not "Drift" racing, he has to basically relearn how to drive, being that his muscle memory is "tuned" for a more straightforward style of racing where being faster is the order of the day, not the flowing style of drift.
Why is Han ripping off the Yakuza despite being a multi-millionaire? It makes more sense after his appearance in later films as part of Dom's crew - it's shown that they are all thieves and hustlers by nature, and Han may have missed the thrill of his old life of crime, especially after the loss of Gisele.
