It's nothing to do with balance. It's all about does the character fit the story. D&D is a story telling game. If a player can come up with a good story to explain why a sentient construct is hunting monsters for the Harpers in the Forgotten Realms they deserve to be rewarded.That DM would either drop that rule or never DM in our group. Full stop.
If something isn’t allowed, that needs to be made clear upfront. And it better be for a reason. 5e dnd just isn’t unbalanced to any meaningful degree.