A little bit of nuance would go a long way.
Even in games where ECS is a good match, having to implement a boss with ECS is wasteful in terms of dev time and performance: You have to create components and matching systems unique to the boss, where with a traditional approach you just have to create a single Boss class.
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.
You could create a database table called "Boss," that contains all the Boss components. Then for each system, you create a function that operates on a Boss.