It is a crap shoot. BMW doesn't treat Dinan any different than any other aftermarket company.
That said, the BMW dealer who also happens to be a Dinan dealer will (most likely) fix any faults and charge the appropriate party (BMW or Dinan) but BMW itself will refuse warranty service on failures they think are caused by Dinan. You are at the dealer's mercy if you break down by a non-Dinan BMW dealership.
Bring your wallet.