Interesting discussion here 
That cool 
Indeed both UCS and NS and BTW any other solution wiling to include Samba 4 will, perhaps with some technical differences inside either emulate Win DC or allow Microsoft server to join as member of emulated domain.
UCS will provide some services NS doesn’t provide yet (perhaps… I’ll tell you more once my platform is up and running).
On the other hand NS offers more network related features.
I don’t think one could definitely state that one fits better than the other everywhere. I share Ctek’s comment on this.
What may make a real difference it admin profile targeted by each solution. There is a balance between very powerful but more complex interface and simple and easy interface but less flexibility.
I don’t really know where NS stands. UCS is quite simple (from GUI viewpoint) but does not permit, as far as I know, stuff like DNS entries that are not A or CNAME records through GUI. E.g.TXT record.