You seem confused about Pokémon roles so I'll explain some of them to you. First of all, Chansey is a wall, not a tank. The difference between both generally comes down to reliable recovery and offensive presence. In Rotom-Wash's case, it's not a wall, but rather a defensive pivot. Unlike pure walls, it can actually come in and grab momentum from your opponent, usually through U-Turn/Volt Switch. And what benefits most from that? Offense. Balance. Not Stall. Another role for Rotom-W is Defogger. I don't think I need to explain this one, or how well Rotom-W performs at it. And then you compare both of them to... Volcarona. I'm not even gonna go there. Also to your last point you mention Nidoqueen but leave out Mamoswine. Yup, offense sucks. Stall king.