No, that's not it. The same problem is true for other error-handling types, such as the "either" or "result" types. Languages without union types or a similar mechanism are unable to define the error types adhoc and force the developer to define them in advance, which is very unergonomic.

But that is true in both cases. The problems with checked exceptions come on top, namely that you cannot use all the regular value-machinery to transform and manipulate the result of a function-call.