Using an entity component system felt like writing very fancy spaghetti code in my limited experience with the Bevy game engine in Rust. No longer having direct references between objects means the type system isn't nearly as useful and I found it very hard to reason about the code. I can believe it becomes useful in very large systems but it was just a hinderance in the small program I was writing.
You can use phantom-typed index types (a wrapper type per container) to recover type safety. Or at least you can when using an ECS in C++; I'm not sure how easy it is to write the wrapper types generically in Rust.
You can have typed indices (https://lib.rs/crates/typed-index-collections), but unless you force the collection to be a singleton, there's no way to prevent mixing of indices between different instances (clones) of the same collection type.
A lot of the mangling produced by ECS is an outcome of representing dynamic behaviors through static optimizations.
If you have a dynamic type that you can attach arbitrary new fields to, then, capability-wise, you have exactly what's needed to make game entities: to do a lookup by entity type, just traverse the list of all entities looking for a magic pattern in the data. To look up a specific one, assign each one a unique ID and search by ID.
The problem is that game developers get anxious about this kind of lackadaisical structure(for good reason, if there's any aim at serious performance) and want to put more things in their own index, and allocate things with a more compact representation and less fragmentation.
And the alternatives are...god object with every possible behavior crammed into an oversized record type, and ECS bookkeeping, in its various flavors, some more compilation-heavy and others more reflective with more runtime editing functionality.
But regardless of what approach you take, every time you introduce dynamism into your entities and enable more flexibility in asset assignment, you convert more of your bugs into data bugs. There are a huge number of bugs in games that are configuration problems with the entity and not a flow control or algorithms issue.
