Nah, NZ are only a couple of points away from England on the test rankings currently, and NZ were ahead and at no 3 in the world about 15 months ago & in that period of from early 2014- mid 2015, were stronger than England overall, so I think it's fair to ask what Williamson's match winnings innings were, if you're going to ask the same question of Root.
The answer is, outside of the Phil Hughes Test when neither NZ nor Pakistan players wanted to be there and there were no bouncers bowled Williamson has only had match winning innings against SL, WI and the minnow teams like Zim & Bang.
I think you seem to be holding Root to an unreasonably high standard that you're not prepared to hold Williamson to.
In terms of the different pitches in India, sure pitches are never going to be the same, but Root in around 9-10 innings, even though he threw away a lot of starts and didn't convert, still managed to average 50, and please don't pretend none of those wickets turned in the games he played in, because 4 of the tests were results. Willamson on the other hand averaged about 25-30 and was Ashwin's bunny. Frankly speaking he didn't look up to the level to face a top class spinner like Ashwin in his own backyard. To be fair on Williamson, they are very challenging and trying conditions for non-sub continent batsmen to deal with, which makes Root's achievements all the more remarkable.
The real question is imagine how amazing Root's tour would be if he didn't throw away those starts when things were looking so easy for him. Guy is a serious talent and deserves to be up with Smith currently, and slightly above Kohli (in tests), and far ahead of the next bunch of Warner, AB, Amla & Williamson, who is further down the list.
The thing you want to look at is their averages against the top nations and that's when you can really appreciate how good Smith and Root are, and how Williamson tend to do really well against weak teams, but fail badly against teams like SA. The one exception was him scoring runs on some of the flattest wickets in australia in recent history, the fact Ross Taylor scored 290 in one of them is proof of that.