Having talked to some of them and having been informed of how they make the list I have to disagree. They take units, strip them of skills and seals, apply neutral natures and analyze performance in a mostly 1v1 context. They also make these comparisons with units performing different roles. Technically this is an accurate way of comparing units (minimizing variance) but it's not really useful in the context of a practical arena setting.
It's why they rate units like Spring Camilla, Ursula and Cecilia so low.
Personally I think this leads to a list that doesn't reflect the realities of the current metagame at all.
I also have to disagree with your second paragraph, even on the highest levels Reinhardt performs remarkably well without support, and only needs one Hone buff to escalate his favorable matchups.
Well, I'm not talking about how the list is created. That doesnt actually matter much. Like I said, Im talking about how to best view and interpret their list. If you just use it to see how units within a class compare with one another, I cant find a lot to disagree with. If you want to compare units across function (e.g., compare how Ike is an S+ and Rein is a S), it doesnt really work, and no tier list could possibly work in this regard, because units can and do serve completely different needs and have different roles. Like, if you need a mage, Effies S+ rating isnt going to mean that you include Effie over a Julia. But you would use an Effie over a Donnel.
Also not sure what you disagree with w/r/t Rein. I didnt say he doesnt perform well; I just said that, to get maximum benefit out of him, you need to use a specific team composition and that limits his flexibility when it comes to team comp (assuming you want to maximize his effectiveness). Unless you disagree that he needs a specific team comp in order to reach peak performance ? I think we can all agree that, while Rein is very strong on his own, he is even stronger on a horse team. And Nino is strongest on a buff team. Etc etc. Units like Nowi dont need much of anything w/r/t teammates in order to dominate, and I think that has value.