Block Hitboxes are too big
Hitboxes are too large, and stops any compact subgrid systems.
As you can see in the picture, the amount of space taken by this hitbox is way too big, around 0.1 m, and cause large seperations for small blocks (not that much for large blocks).
For exemple, the new block, hinges, generate lots of phantom forces and need clearance around it to make it work correctly.
It's simply a bad idea to use subgrids in a build, due to not being easily built in survival, but is dangerous too.