There's an FHWA Manual CFLTD 06-006 on rockery. I also attached a standard drawing from 1913 (NYC Bronx Borough President) on construction of dry rubble walls.
I've inspected several hundred rubble walls, quite a few in the 20+' height. Bulging is obviously a major concern. I've seen concrete walls that have noticeably tilted or displaced laterally; with stone they'll fail first. Another thing to look for is weathering/deterioration of stones; erosion along the base because water is meant - at least in the old days - to flow through the wall. Vegetation growing from a wall can also affect stability.
Most of the stone walls I've inspected are probably 100+ years old and generally still functioning well.
Attached is a rockery design guide. It sounds like they have preformed well however I still feel slightly uncomfortable with them. Usually the ones I see are missing stones/boulders or need more frequent maintance. However these are probably not designed or not constructed properly.
