[throwaway894345]

Type safety And query semantics, mostly. You can use constraints to get some of this back, but it only goes so far. Ultimately there’s a reason statically typed programming languages developed sum types, and all of those reasons apply to databases as well because data is data.

[gfody]

you can avoid nullable cols by using separate physical tables for each concrete type with surrogate PKs from a common sequence. these can be concatenated back together in a view (with no performance hit if you're careful not to hide indexes and predicate pushdowns). FKs then can reference the appropriate concrete table and in case you need an FK on some union you can use an indexed view instead.

I've used this approach for modeling entities that are polymorphic in object-land and haven't felt particularly underequipped - more robust indexed view support would be nice but that's active research territory (one of the hardest problems in computer science, materialize.io looks promising!)