Al lot of the same properties as a mine field. An unobserved mine field is quite quickly passed through. The issue primarily arises when you need to clear a mine field while under fire.
Combine them with ditches, barbed wire, possibly mines, and have forward observers directing fire on anyone trying to break through, and they’re a real PITA for attacking forces.
Essentially, they give just a couple forward observers the power to hold up an attacking force for quite some time while reinforcements arrive, at which point you’ve lost whatever advantage of manpower or surprise you had when attacking.