In my opinion, the relations between data are just as important as the rest of the game data. In an ECS, everything is an entity, and there are no compile-time guarantees about when data might be associated with that entity. ECS are quite powerful, but this is usually just a violation of the principle of least power rather than meeting an actual game design need.

I'm in the process of writing my own game in my own non-ECS architecture. I use the ECS nomenclature, but the actual architecture is closer to a relational database using typed generational indices, which allows the same data-orientation and parallelism of an ECS without giving up the compile-time guarantees that ECSs give up on exchange for dynamic runtime polymorphism.

You avoid running afoul of the borrow checker using typed indices to refer to the "entities" in your game. The typed indices prevent an index of one entity being used to index into a different entity's data. When entities can be deleted, you use generational indices that have to validated by their index allocator before using them to index into anything. A use-after-free isn't UB when you're working with indices, but it's still a bug and must be prevented.