The node does provide meaningful reject reasons since 4.1

[abizjak]

Task description

In the README it states
"submit: Fully implemented. If the node rejects the transaction, an error with no details is returned. The server could test for a few possible reasons (validate signatures, check balance, etc.), but the node itself doesn't provide any explanation for the rejection."

which is no longer correct. Since at least 4.1.1 the node returns a description (as an RPC error) of what went wrong if the transaction is not accepted.

This could be returned to the sender/user of the rosetta API.