Hairline cracks that run in random, spiderweb-like patterns and stay under about 1/16 inch wide are almost always cosmetic shrinkage cracks from the stucco curing. They are extremely common, do not indicate a structural issue, and get bridged and coated as part of a normal repaint.
Diagonal cracks running from the corners of windows and doors are the next tier. These usually come from minor settling or seismic movement concentrating stress at those corners. They are worth watching but are typically still a prep-and-fill situation rather than a red flag on their own.
Stair-step cracks that follow the mortar lines of a block wall, or wide horizontal cracks along a foundation line, are the ones we stop and flag. Those patterns can indicate actual structural movement, and painting over them without addressing the cause just hides a problem that will telegraph back through the new coating within a season or two.
Our practice is simple: hairline and diagonal cracks get elastomeric patching compound and painted through as part of the job; anything stair-stepped, wider than about 1/4 inch, or paired with sticking doors and uneven floors gets a referral to a structural engineer before we touch it with paint.
