The main issue is that
- they choose the nearest target and as a result take a walk in unexpected directions. This could be fixed by adding a distance penalty for other directions so that the breaker is most likely to pick the building (and walls in its path) even though that building is a little further away but in its expected direction
- they still target disconnected walls often, and not proceed to enclosed sections with buildings inside
- it looks like the detonation speed of the breaker has been nerfed, more then ever they got shot before they had the chance to detonate near the wall
- they only proceed to the next closed section as long as the previous section got cleared from buildings. By the time that happens your troops are stalled untill you get the opportunity to break the next section. Very inneficient.