I would say it is comparative in terms of price between the two systems when controlling lighting systems. Perhaps, Niko is slightly more expensive but as the system is permanently linked to Niko's servers it regularly gets system updates with new features. I am only referring to residential systems here - I have no experience of other commercial applications.
When you say 'closed' do you mean being limited to the the number of zones/channels on each Lutron control unit? Then no, as Niko is a modular system. You buy your wiring panel and fit whatever modules you would like (not limited to lighting). If you mean, do you have to use all Niko control kit? Then yes.
By the way I'm don't have a vested interest in Niko (or Lutron) just an installers opinion. I must confess I haven't fitted Lutron for a number of years as they seemed a bit behind the curve, so it may be all different now.