To me, Haynes manuals are just that.
As for correcting errors Nick Hopkins, MD of Andover Norton and Commando owner, tried to re-write the 750/850 manual 06-5146, because he thought some passages in it were incorrect resp. could be bettered. For that reason we postponed a reprint for over a year. In the end I ordered a new print run of the old version because we had lots of orders but nothing to offer, and it became obvious that Nick, with his normal work load running Andover Norton, would never come round to finishing the revision.
I have worked with the original version since 1977 and if that was good enough for my workshop personell and myself over the years I thought it will do for the average private owner. And it is head and shoulders over most pirate manuals. However, there are exceptions. If you look into our "book list" under
http://www.andover-norton.co.uk/Books.htm, you will note I have underlined the Chiltons and Clymer manuals as a "must have", because I think in some respects they are very good,
AND they contain some information that the original manuals don't give, like the differences in balance factors etc.
As for other manuals, on the Triumph sector in particular I notice immediately if customers work with a pirate rather than the original Triumph workshop manual, because these customers then try to order or install bits that their Triumph model never had- because the author of the (Haynes/Bucheli) manual obviously knew nothing about the technical changes over the years. Which may be the reason why our original Triumph 650/750 unit twin manuals- of which 4 different ones for the different model variations exists, but Mr.Haynes tries to condense that information into one manual with less pages- are the best-selling titles in our portfolio of genuine Triumph technical manuals.
The same applies to the Commando manual, though in milder form, as the changes were either minimal or, as in the case of the 850Mk3, so obvious even the layman sees where the Haynes manual is "inaccurate".