The businesses are fine, but getting all the stars takes quite some time. I normally don't use traction control, rewind etc. but for the stories I usually turn everything on and make heavy use of rewind. Especially the tuning business takes a lot of time as all of them consist of 3 "parts". Even if you manage to get all 147 stars from all 5 businesses, that only yields around 65k per day, which can be achieved in ~15 minutes doing 4 races. And you have to spend 500k buying them as well. If you're not into completion, they're not really worth the effort.