I don't craft mines but it looks like the active 'wire' bits are not voxels like most other things in Minecraft, but instead are painted on 2d surfaces yea? Please elaborate!

They are voxels, they just have very specific placement rules. They take up a full block but can't be stacked.

There's also the fact that the wires transmit information only 15 tiles. It's possible to build repeaters though, but those repeaters have some lag and take up space.

The hardest part are the memory cells. A single bit, even in the optimal configuration, takes up 3x3x2 blocks.

Correct. They can only be placed on the top of voxels, but they can propagate forward and up. I.e. you can build "stair wires" but not "column wires".