It's a bit like being a running back in the NFL... you could have incredible attacking potential, but no team will let you on the field to play until you've proven that you're at least serviceable at handling the un-flashy, team-first fundamentals like pass blocking. If you can't do that, then the team suffers even if you manage a few big plays - fullback is exactly the same.
It's why we've looked so much better with TMM out the back despite him lacking the attacking flair and playmaking ability of Niu - and it will be the same when Walsh comes... I don't want him in that 1 jersey until he's shown that he is capable at being in position, organising the defensive line, getting involved in holding up players over the line etc etc. At number 1, you're a spine player, not just an attacking weapon. How many gun attacking players have we seen that aren't able to develop into good fullbacks - Dufty, Savage, Sloane...
Niu has a ton of attacking flair and potential - and that's why some posters here are so high on him, but has shown very little in the more selfless, team-centric roles of being a fullback - which is why others don't see him as a viable option. He'd probably be better off in a position like centre where he could attack without the burden of being a spine player, but the consensus seems to be his defence isn't of the standard to make that viable.
Are those team-based, spine player responsibilities / skills something that can be taught? Yeah I think so - certainly more teachable than natural attacking flair and ability... but they've got to be dedicated to improving and learning. They're all young kids, they'll have ups and downs, but you want their focus to be on steering the ship and defence first, and then attacking can come later when they're comfortable with the fundamental responsibilities.
The fullback battle next year might not come down to talent or attacking / playmaking ability, but just whoever has worked harder to become a more complete spine player and do the behind-the-scenes, fundamental things that drive the team - even if that brings the absence of huge flashy attacking highlights.