UDK pretty much is a must, yes. He goes in a lot of teams for me due to the super stoneskin stall meta. Have to outstall the Rotoses too, before they get off 5 extra turns and wreck you. The Go Third meta is soooo time consuming.
Ninja is good in Normal Hydra, where Defense isn't as terrible, Hydra max HP is still moderately low, and his low HP builds don't cost you death after death. He falls off to, imo, okay at best in Hard hydra. I'm still of the opinion he is a lot less of a must build for a late game player who has solved DT and has a Geo, a Husk or Royal Guard or even, here's hoping, an Acrizia. In the same slot, I actually see a great deal more damage with Big'Un than Ninja on Hard hydra, myself. :)
Early-Mid game Ninja is great. Eventually, in the toughest stuff, he falls off in favor of Reflect Damage and Enemy Max HP. UDK is much better in Late Game than he is in Early Game. Maybe he'll fall off one day as new champs come out, but I don't see that happening any time soon.