I need to understand the text down below, but have no idea what the last sentence means.
Specifically, what does "abuse" mean in the context?
Also, "success is limited at best" what does it mean?
Please help.

I don’t want to leave the reader with the impression that the biomedical and behavioral approaches were unhelpful. As I have noted, they often benefited patients who were in terrible situations. Yet they routinely failed to examine adequately the issues described in this book. To my mind, that failing significantly limited the value of the care; it led to certain predictable abuses and contributed to a dangerous claim of near- omniscience regarding a health problem for which very little is known and success is limited at best.