It is curious to me how with Volc people always says that it picking its counters was the worst for the competitive scene but with Valiant doing the same they said "well, it can pick its counters but can't beat them all with the same set" which for some reason it applies to Valiant but when you said that about Volc it wasn't a valid argument.
The biggest reason I support Valiant but hate on Volc is what the moth can do outside of tera on top of it's offensive sets. Valiant can be either physical or special and can run mixed sets, but it's frail, has difficulty setting up, and if it's forced out after the use of Booster energy then it's a major momentum loss.
Volc has no such issues, but on top of that it has 2 forms of reliable recovery, the best set up move in the game, and a Unique Fire type move which in this meta is a blessing because you can proc a special attack boost at the cost of the extra initial damage other fire moves provide. It also has Bulky sets which can opt to Burn the opponent. And since it even has Flame Body, just attacking it with a physical move can cause burn (contact, but you get the idea). And keep in mind, I haven't touched on Tera yet.
Valiant imo is just to much of a glass cannon. It can pick it's threats, but unlike Volc it can't adapt to the situation without good support. And while Valiant has an amazing ability with booster energy (cause no way your getting electric terrain up), it is reliant on a single use item. It's functionally an Unburden user in that regards, great when it works and decent otherwise. But if your forced out, you lose major momentum. But not with Volc, you can keep bringing it in since it's nearly always running Boots. Sure you don't like to give up your Quiver dances, but with Giga Drain and morning sun at it's disposal it can outlast and potentially reset up.
Valiant is an extremely good pokemon, but volc can ultimately pick it's match ups better because it's Grass/Fire coverage moves hit basically everything not named Heatran for neutral. So if something it can't deal with comes along, Tera Blast becomes stupid good. And while Valiant can run Tera blast, it has such a vast array of coverage moves that are superior. Therefore, while it can punch holes through teams, it can't systematically destroy its checks with it's Frail bulk. Valiant can't reliably run a "defensive tera" because doing so might still get it Ko'd, so instead it runs the Tera type that works best with the offensive move it wants to use. Thats why Fighting, Fairy, Ghost, and Electric are such common Tera types. Because Valiant has much better moves than Tera Blast given to it and doesn't want to have a 80 BP normal move that may or may not have the correct Tera for use each game.
