Some osnaps (e.g. NEArest and INTersection) do not work well (are not displayed) on some types of entities nested in blocks inserted with unequal X-Y-Z scales.
If your non-uniform block reference contains arc (incl. polyline arc segment), circle, lwpolyline (incl. rectangle, polygon), ellipse, xline, ray, mline, or leader object, the object snapping feature doesn't work on these objects embedded in the block.
AutoCAD versions also have problems with NEArest osnaps to circles and arcs (incl. old polyline arc segments) not parallel to the current UCS.