The AGBI has a lot of speculation in it, not identified as such. Such as attributed dates of birth based on purported service of a same-named person in the Revolutionary War. I had a protracted discussion with someone who found such an entry and was fussing about finding verification of the date - she did not realize it was just made-up by the compilers.
There are a great many such compilations, the near-worst of which are extracts from family group sheets, from non-researched published genealogies, etc. The worst are published deliberate fictions, or those that are largely fiction but salted with stuff extracted from secondary-source versions of actual records (e.g., the compilations by Virkus). There are a lot of these living amongst the Ancestry.com databases.
The essence is that you must use your brain to evaluate a source's quality. When it gives you no way to track to an original source, occasionally the assertion will prove to have been correct - but you still must do your own research where the actual records are, to find actual evidence.
Do take a look at a recent blog post by James Tanner, concerning the distinction between a source and evidence:http://genealogysstar.blogspot.com/
A more in-depth outline can be found here:http://www.bcgcertification.org/skillbuilders/skbld085.html
So many, these days, are not taught how to do evaluative reasoning ("how do I know what I think I know?"). It can be a big step to start to get comfortable with evaluating sources and sorts of evidence. Once you really get into it, you will find that there is lots of background and history you will want to know about. Happily, there are many resources for this on the internet these days, and even this vast body of stuff is only the tip of the iceberg compared to what else is in the treasure-houses (Archives, libraries, historical societies, courthouses and church records repositories, for instance).
So, use your brain and go for it :D