Drive around Mount Penn or any of the hillside neighborhoods in Berks County and you'll see the same failure over and over: a block wall bulging in the middle, tipping forward at the top, with the courses no longer lining up. That wall didn't fail because the block was bad. It failed because it was a drainage project treated as a stacking project.
Soil behind a wall gets heavy when it's wet. Saturated backfill can exert several times the pressure of dry soil, and that pressure has to go somewhere. If there's no clean drainage stone behind the wall and no perforated pipe at the base to carry water out, the wall becomes the dam — and walls make poor dams.
The base is the second failure point. A leveling pad needs to be excavated below grade, filled with compacted aggregate, and compacted in lifts rather than dumped and raked. Skipping this saves a few hours and costs the whole wall, because the first course sets the geometry for every course above it.
Then there's batter and reinforcement. A wall should lean back into the hill slightly, not stand plumb. Above certain heights and in certain soils it also needs geogrid — layers of reinforcement extending back into the backfill that turn the soil mass itself into part of the structure. Whether you need it depends on height, slope, soil, and what's loaded above the wall.
Which leads to the honest part: in Pennsylvania, walls over four feet generally belong in the hands of an engineer and often need a permit, especially with a driveway or structure above them. We build plenty of walls below that line, and when a job crosses it, we say so rather than building it anyway and hoping.