He's really unsteady when things are not in the right track, and he clearly lacks the mental strength to lead a team when things are getting tough.
Last season was smooth for Sheffield United, so Lundstram's weakness was not easily exposed.
For the current one, which has been chaotic, he's looked notoriously shaky and unstable. I still think he's an intelligent player by default, but his mindset gets easily overshadowed under pressure. So he eventually fails a lot under those circumstances, with erratic build up play end up product.