The thesis is, in general, good: It is epistemologically possible and sound to have (some) general rules about the probative worth of (some) classes of evidence.
Schauer's general thesis is, thus, sensible. But more arguments in favor of his general thesis must be made. For example, one might consider how it would be possible to learn from experience if one could not extract (whether implicitly or explicitly) from experience any general principles about the workings of the world and the relationship of events in the world to phenomena that seem to serve as indicators or signs of events; complete "individuation" of judgments about probative value (a/k/a evidentiary value or force) would seem to bar the possibility of knowledge based on experience.
A separate (and important) question is whether the the particular generalizations that are or may be embedded in the American law of evidence about the probative value (or lack of probative value) of certain categories of evidence (e.g., hearsay) are warranted. The mere fact that there must be some generalizations does not mean that American law has identified the correct ones. Still, the argument made by Schauer is refreshing. It is the beginning of a sensible attack on the ludicrous (so I would say) hypothesis that the probative value of evidence depends entirely on individual circumstances and details.