We're kind of beating a dead horse here, but I think where people probably get into trouble with cinder blocks is when they are used "naked".
Without something else to spread the load, having an axle or frame member bear directly on a small portion of a cinder block is just a recipe for disaster.
With the load properly spread, a cinder block is probably capable of bearing a large load. With the load concentrated on a small area, not so much. The trick here is "properly spread".
Best approach is to just stay away from them. It's particularly important to discourage a first-time backyard mechanic from using cinder blocks.
I'll get off the soapbox now...
Z.