The National Foundation Repair Association is a trade association for foundation repair contractors. They publish a document titled Expectations of Underpinning. The entire article is worth reading, but this page focuses on the last section titled: Limitations of Underpinning.
Movement outside the underpinned area
There is no promise or warranty that any area outside the underpinned area will move or not move enough to cause visible distress or damage. It is not uncommon for a homeowner to see some distress in a particular area and conclude that if he or she underpins that area everything will be okay. It is not uncommon for new distress to develop in adjacent areas that were not underpinned.
There is never any warranty against heave, only settlement. The difference is this: settlement is a downward movement while upheaval refers to upward movement. The reason there cannot be a guarantee against upheaval is that repair piles and repair piers cannot restrain upward movement.
Interior floor instability as a result of interior settlement and/or perimeter upheaval
If the interior area of the slab foundation becomes unstable and settles more than the perimeter, it can have an adverse effect on the frame structure. The walls perpendicular to the exterior perimeter walls can be lifted off the floor. This manifests itself as s separation between the floor and the wall or a separation between the wall and the ceiling.
All of this can be a direct result of the decision to not use interior pilings.
Damage From vegetation
Foundation performance can be adversely affected by vegetation, especially large deciduous trees. The situation can be made much worse if the foundation perimeter is underpinned. When one of our recurring droughts shows up, the trees will draw more moisture out of the ground underneath the slab than normal. The interior portion of the slab drifts downward while the perimeter is held up by the previously installed piles/piers.
Point of Contact
In a normal slab foundation repair, each pile or pier should be in contact with the bottom of the grade beam. In fact, the document specifically says:
Underpinning is only as good as the contact or connection point between pier/pile and the structure. If the grade beam, thickened slab, or steel beam support is faulty, pier support will not be fully transferred to the foundation and downward movement may occur.
Even if this connection is initially perfect it is highly likely that it will degrade over time. The performance of the underpinned slab will also degrade over time.
Consider the most popular type of underpinning: segmented piles
Repair contractors love this system. They cost much less than the traditional bell-bottom piers. segmented piles typically cost around $350 each and bell-bottomed piers run around $550 each. They supposedly result in fewer call-backs from complaining homeowners.
The downside of this system is that the driving force is the weight of the slab and the house. It is possible that the stable soil is at 14-feet and the pile will only go to 3.5-feet. In that situation, the money spent is wasted.