The concrete blocks in the image above appear to me to be sections of sidewalk that have been stacked together! Clearly a great way to recycle materials, but not a particularly aesthetic one. These blocks don't have the mass that basalt boulders do, but they should allow drainage because of the gaps in them. While the gaps can allow weeds to grow, it's not as great a problem as with the often large gaps found in boulder sized stone walls.
Because concrete can be cast, many shapes of concrete block can be created which can then be stacked together. It's not hard to find walls made of ornamental concrete bricks that create walls just a few inches thick, neither is it hard to find stacked "Jersey Barriers" that are at least two feet thick!
Because casting a slab involves extensive use of temporary forms, one way to avoid this is to start with "breeze blocks" which are concrete construction blocks that include holes on the inside. The blocks are stacked like bricks in overlaping rows, then fluid concrete is poured into the empty columns that the stacked blocks form. Rebar can be included to further increase the strength.
The wall shown below is similar to the one seen above though I suspect it's older, due to the discoloration. The designer pitched the wall into the earthen face, as can be seen on the right side of the wall. You can see how weeds have begun to infest the wall. To its credit though, the wall has maintained its physical integrity.
The terraced concrete block wall shown below is an excellent example of fit and finish of concrete blocks. Obviously these are not simply broken sections of concrete sidewalk stacked together, but commercially formed blocks. The bottom row of large "breeze blocks" seems to have either mortar added between the blocks, or fluid concrete poured into the vertical channels formed by the offset blocks. I hate to be a pessimist about something that looks so nice, but I'm not sure how well the smaller stacked blocks will resist hydraulic forces over time. I doubt things will look like this in twenty years.
