While I don't play Ubers much, I can see a few qualities in Reshiram that could possibly keep it out of D rank. Firstly it can check Ho-Oh, being able to switch into Sacred Fire for days, and being able to live non CB Brave Birds, while being able to take it out with Stone Edge. Plus being neutral to Fairy is nice if you need an emergency check to LO Xerenas.
It literally switches into Leftovers Ho-oh(arguably the least viable set) once, and even then it's not loving life:
0 Atk Ho-Oh Brave Bird vs. 0 HP / 0 Def Reshiram: 162-192 (47.5 - 56.3%) -- guaranteed 2HKO after Stealth Rock
Plus if you switch it into Ho-oh you defeat the purpose of running Stone Edge because any half competent player will realise what you're up to.
On the subject of LO Xern, yeah I guess it can revenge kill it but it's certainly not coming in on Moonblast even with a Fairy neutrality.
252+ SpA Life Orb Fairy Aura Xerneas Moonblast vs. 0 HP / 0 SpD Reshiram: 255-302 (74.7 - 88.5%) -- 93.8% chance to OHKO after Stealth Rock
I don't mean to be harsh cos I really like Reshiram design-wise and wish it was more viable, but honestly, there hasn't been a single team out of the many decent and not so decent efforts I've made this gen where I've ever considered using Reshiram because there's always been something that did it better. I've also tried building around it, but when you actually look at it, it doesn't have that many positive traits for you to work off. Sadly I think it belongs in D Rank.