The nerf to sunlight's time, along with Assault Vest might make things difficult for Reshiram somewhat. It's Flame Charge set is harder to use now thanks to Sun dying out faster (the Tailwind set could have trouble too, though that often is short, and still can do things like let stuff outrun Geomancy Xerneas). Assault Vest can be annoying on stuff like special defensive Dialga, as it could tank your Blue Flare even in Sun, then KO with a Dragon move. It's two key dragon moves also got a nerf as well. There's also the fact it may compete with Ho-Oh for a team slot, and the latter more or less walls Reshiram due Defog (although Defog also makes it easier to use both of them together on a team!)
However, the fact that it can actually switch in more often now thanks to Defog will allow it to fire off Blue Flare's more often, and 8 turns of sunlight should be plenty of time for it. Reshiram itself might also make for a good Assault Vest user, as it will help for opposing special Dragon moves, surfs from Kyogre, and Fairy attacks (Geomancy Moonblast will still hurt, although it won't KO. Reshiram can't do much in return though thanks to boosted Special Defense). The Sunny Day set is now more interesting due to Groudon not being able to provide Sunlight as easily.
I find it a little hard to tell if Reshiram will will be better or worse this gen. I'll probably do more Ubers matches with it; and see how it goes. Not being weak to Fairy, and being able to actually switch it in more often should help a lot.