There are certainly fraudulent genealogists (Gustav Anjou is the prime example https://www.wikitree.com/wiki/Category:Gustave_Anjou_Fraud )
There are also authors that make mistakes and others that go into the realms of wishful thinking, often wanting to link to some aristocratic line.
Whatever source you look at you need to ask whether it is credible? Does the author reference where he got the information from? Does it hold together well; ie why does he suggest John is Fred and Mary's son. (I find that there are a lot of individuals who are linked to someone on the other side of the Atlantic with nothing more than a name in common) Fortunately we have good access to many more records than these early writers did. It is often, possible to check their veracity.(I've been doing that with a book that claims a man was Lord of a Manor when he wasn't and that other members of the family were clergy with named dates/places of degrees and clerical positions held but University records don't include them and church records show other men held those positions at the time. )
I had a look at the book you quoted. I took the first name of an emigrant to America . This was one James Cade who he said settled in Hingham Massachusetts in 1635. The book says he was baptised in 1611, the son of Chrisopher in Northam, Devon and married a Margaret Browne in Bidford (Bideford), Devon in 1633 with a son Philip born in Northam in 1635.
There aren't any images of these early registers online but there is an index to them (IGI) on family search. All three of these events are indexed (last is a baptism, not birth)
But I'm always suspicious, how do we know that James Cade of North Devon went to America? The author refers to a mortgage document concerning land in Northam, signed in Boston in 1638, he also says James was a shipwright (this rings true since Bideford is a port and has had a shipbuilding industry) He also references this citing 'The Book of Thomas Lechford' p 42. Google finds this book " Note-book kept by Thomas Lechford, Esq., lawyer : in Boston, Massachusetts Bay, from June 27, 1638 to July 29, 1641'' on the LDS library website . On P42 there is a transcript of the document including far more information than given in the Allen-Palmers Allen's book. This document has a lot of local knowledge (I know that because it names the village I was married in) I would say that there appears to be good evidence (unless the notebook itself is a fabrication) that this James Cade went to the New World in 1635 .
So thumbs up from me for the information given on this James.
That doesn't mean the whole book is credible. If you think that part it is fraudlent, or in error, to use a milder term, then do the same thing as I did; check the information given. The fact that he does reference his work makes this far easier than a lot of books from the era.
edited author's name