How reliable are foundation performance evaluations?
This is a very interesting question, but it is not clear what the answer is. In fact it is not clear that it is even possible to answer the question. We really do not know how reliable foundation performance evaluations are and, I would argue, it is not possible to know how reliable they are. It is not possible to know, at least in any verifiable, quantitative sense.
Foundation performance evaluations are always subjective opinions.
The subjectivity makes many engineers and some real estate inspectors uncomfortable; but there is no way around the fact that these evaluations are full of subjective assessments and opinions. For instance any recommendation to underpin or not to underpin a foundation rests, at least in part, on a subjective assessment of the likely effectiveness of underpinning and it’s associated risks for a specific foundation.
The decision to repair or not repair a foundation is ultimately a subjective judgement call
For a specific foundation, there is simply no way to know how effective foundation underpinning will be and what the costs (in terms of damage to the foundation and the house) of the underpinning process are until after the foundation is underpinned. In fact, it will normally be some time after the repair work before a reliable assessment can be made of how effective the foundation repair was. And, if the foundation is underpinned, we will never know how the foundation would have performed without the repair.
Why you will never know if you made the best decision
In a sense deciding to repair or not to repair a foundation is like a fork in Robert Frost’s road in his famous poem The Road Not Taken. There is simply no way to ever know for sure if you made the best decision or not. For precisely that reason, it is important to gather as much information as possible and come to the best understanding you can before making a decision as to how to address expansive soil foundation problems. One essential element is to seek the council of an unbiased structural engineer who specializes in this area.