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they are probably just as accurate as any other typical record
look at your own birth certificate and see how many errors may be on it
or your parents death certificates.
just because a document is a primary record - doesnt mean it doesn't have errors.
How error prone a given set of specific documents is - as you ask - is very difficult to gauge - but you have to assume it could be wrong - and you collect as many OTHER different sources as you can to see how they together support the facts.
there are various videos by Crista Cowan and related subjects you can find on You Tube