Sometimes bouncing a ball off the rail can produce an odd collision. Besides that, the physics work well enough.
As for the economy, sometimes you will get ball options that aren't worth the money. For example, on several occasions I've seen a basic ball that costs more than it pays out (example: it costs 4C to buy but it only pays out 3C ). Why would I buy that ball when I there are usually options where the ball pays out more than its cost (or includes an effect)?
Hm I see. That should have been in later rounds on theory. Everything gets a bit more expensive over time, purchasing the right balls in the beginning is already a bit important. Balls costing more than they get you can definitely happen, sometimes, with abilities, also in very early rounds. They stay for five rounds so they'll make you money up to five times