Shears need a bit of special treatment so that they're held in a way that makes sense.

In the main hand they need to point towards the center of your field of vision, and should probably have their gripping point adjusted.

In the off hand they just need to have their gripping point adjusted.

Update: I think it would be better to solve that with an resource pack, rather than hardcoded flip the model. That should be the same way swords etc are done.

Noooot sure there is much I can do about that right now. I guess turning it sideways would look better? https://github.com/tr7zw/NotEnoughAnimations

Yep, turning them so that they're vertical rather than horizontal would be much better =]