The problem with making invalid states representable is that the app logic must be able to handle it, everywhere. Which means you have to always take it into account when reasoning about your app and implementing it, otherwise it will blow up in your face when the invalid state actually occurs. Which means your state machine is inherently larger and harder to reason about.
To illustrate what I mean, take the null pointer. It's essentially an Optional<T> and in a lot of languages it's involuntary, meaning any value is implicitly optional. What happens when something returns null, but in some place in your logic you don't take it into account? Null pointer dereference.
Yep. That's what I was going to comment.
I'm not convinced, because you have to deal with invalid state in some way or another in any case.
If you have to delete some record referenced by other records using foreign keys, you'll have to handle the complexity in any case, except if this is enforced by the database, you'll have no choice than to think about this upfront, which is good. It might lead you to handle this differently, for instance by not deleting the row but mark it as deleted, or by changing the id to some "ghost" user.
If you don't do this, all the complexity of not having to deal with the inconsistencies when you are creating them will have to be encoded in your code at write time, with null or exception checks everywhere.
Constraints encodes what safe assumptions your code make to be simpler. If an assumption needs to be relaxed, it's going to be hard to change things as you'll have to update all the code, but the alternative is to make no assumption at all anywhere, which is hard too and leads to code that's more complicated than necessary.
And then what does an invalid reviewer_id mean? Was this caused by a user deletion? Or a bug somewhere? Or some corruption?
Bonus: some commenters here write that it can depend on which programming language is used, I don't think it matters at all. You'll have to handle the null or dangling values in any case, whether the language guides you for this or not.