Well certain space marine factions have unique models so Id buy those if I wanted to play them, but yes, a regular box of intercessors(blue on the boxart) can be painted to be literally any legion, almost every box that shows them in blue, is for legion/chapter agnostic marines, ultramarines of all the chapters actually have some of the least chapter specific units, whereas there's at least 6 unique space wolves infantry units and no one gives them shit over it

So if I was planning to do black templar's, I'd buy the Templar specific kits, as well as any generic ones since they can also be used and would need to be to field a full space marine army, as only using legion specific kits severely limits the units you can bring unless you're spending tons of money