Yea. This has been a very common bit of feedback. I'll try my best to break down how they are placed in game and the reasons we are not yet able to make any block a slab for visual reasons.
The slab is a full block, but squashed. This means the textures on the side of the block are still a full square, then squashed to 1/2, 1/4 or 1/8th of the size.
With Cargo blocks we purposely designed textures to counter act this squashing effect. It works great, but remember that cargo blocks don't rotate.
When a block is rotated in Starmade, the textures do not always stick to the same face, so the "squashed" textures will flip onto the un-squashed face, making everything look terrible.
Why don't you just let the slab squash the texture for mesh / grill / scaffolding?
I considered it, for sure. But it's not pretty and i'd really like to provide these options when rotational problems are fixed and looking correct.
Thanks for the suggestions!