The variables in use for a mobile tuner are so infinite that nailing performance down with a few numbers is next to impossible. This is especially true when it's understood how the measurements are made and how they represent the real world. The tuner spec problem is really a matter of there being a problem with the measurement duplicating actual listening conditions. Because duplication of this exact condition is not possible, the spec can never be considered more than a general guideline. However, this is not the only problem with specs.
Some specs suffer from other shortcomings.
For example, some specs are very easy to quantify, but their real importance is overstated. Distortion, for example, is a very important spec. It can tell us about how well a component is designed. Over the years, components have gotten so good that the distortion spec has far removed itself from real perception. Today, numbers on the order of .00 are common.