While projects will list "reliable" sources because they haven't been found to be UN-reliable, you should evaluate a source for every statement that it is making. Someone could have info for a person in a source that is hearsay and incorrect, while the rest of the entire volume could be correct. It really is a tricky business, and likewise something that someone says is "unreliable" because their family line information isn't correct could have correct and reliable information on yours.
Use the Genealogical Proof Standard, think about whether the source is primary/secondary/derivative/proximity to the event, all those things when evaluating whether to believe a source. If you can't tell, but you have a document that says "XYZ happened", then assume it is correct until other documentation refutes it. Even family legend can be correct and point you in the right direction to find other sources to corroborate.
