Hacker was a pioneer in writing about pipes and his interviews with Sasieni are unique. But, like a lot of pioneers, he got some things right and some things wrong, and current scholarship has long since passed him by. John Loring devoted a page to listing Hacker's errors regarding Dunhill pipes, and most of what he wrote about Barling was wrong.
In his defense, he was writing before the internet was anything like as robust as it is now, and a lot of material that has turned up in the succeeding years would not have been easily available to him, though huge amounts of information, that he clearly didn't consult, was, and still is, available at the NYPL and other libraries.
My chief problem with him is that he never says, "to the best of my knowledge" or "as far as I know". He states everything in absolutes. Among my favorites is his statement that Barling never published a catalog prior to their 1962 150th Anniversary edition, which isn't true. Close to a decade ago, I published a facsimile of Barling's pre WW1 catalog. So while I enjoyed Hacker's books and have kept my copies, I don't consider him an authoritative source.