We get called to look at leaning and cracked retaining walls across the hillside neighborhoods every winter, and the diagnosis is nearly always the same. Nobody skimped on the block. They skimped on the four things you cannot see once the wall is up.
The first is the footing. A retaining wall is a structure, and a structure needs a poured, reinforced footing sized to the height it is holding, below grade and on undisturbed soil. Block stacked on a scratch trench will move the first time the ground gets saturated.
The second is reinforcement. Vertical rebar tied into the footing and grouted solid inside the block cells is what lets a wall resist being pushed over. A hollow, ungrouted wall is a stack, not a wall, and it behaves like one.
The third and fourth are drainage, and they are the ones that get skipped most often. Soil is heavy. Saturated soil is far heavier, and the water trapped behind a wall exerts hydrostatic pressure that can push over a wall that would otherwise have held for decades. The fix is gravel backfill against the whole back face, and a perforated drain line at the base running to daylight so winter water leaves the site instead of sitting there.
None of that shows up in a photo of a finished wall, which is exactly why it gets cut from cheap bids. When you compare estimates, compare footing depth, rebar spacing, grout, backfill material, and where the drain line goes. If a bid does not mention them, it is not a cheaper wall, it is a different and shorter-lived product.
On the steep lots we work in Los Gatos, San Carlos, and Belmont, the other thing worth knowing is that two or three lower terraced walls are frequently safer and better looking than one tall one, and often cost less than engineering a single wall to the same height.