The bug

You can place buttons, torches, etc against the sides of snow blocks, but not against the sides of a full block's worth of snow layers.
Similarly, fences and other connectable blocks will join to the snow block but not the layers.
As this doesn't seem to be a deliberate exclusion, I thought it should be raised as an issue.
It's also worth noting that in 1.12, you can't place a button on the top face of 8 layers of snow, which is something that was possible in 1.11.

Code solution

Changing BlockSnow#getBlockFaceShape to the following would make it's behaviour consistent with other 'full' blocks:

public BlockFaceShape getBlockFaceShape(IBlockAccess worldIn, IBlockState state, BlockPos pos, EnumFacing face)
    {
        return face == EnumFacing.DOWN || state.getValue(LAYERS) == 8 ? BlockFaceShape.SOLID : BlockFaceShape.UNDEFINED;
    }