On the other hand, the classical approach lets you treat the boss as the one-off actor of an one-off scene, which it really is. This narrows the scope of your work (and associated testing). Integrating it into your overall ECS means you either make all aspects of a boss fully generic, working everywhere, or risk someone at some point adding "SpecialLaserCannonHelmet_Boss1" component to a random mook and breaking the game.
It wouldn’t break the game, it just probably wouldn’t render correctly since a boss works with certain graphics.
Having a one off actor is really limiting. You can create several boss components and build entire permutations of bosses just by mixing and matching them up, without writing specialized code. Designers can put together the entities without developer input.
