In my experience, the finger scanners are very accurate.

Long story short, my Dad and I accidentally exchanged cards during a meal (we were both redeeming credits from a meal plan and both have the same name). Anyway, we split up after the meal, with my Father and Mother going to the Magic Kingdom while my wife and I headed to the Studios.

Obviously my Father's finger would not authorize my card for entry into the Magic Kingdom and after some discussion with Cast Members and showing his ID, the Cast Members "reset" the card (my card) to his fingerprint. A few minutes later and after comparing that card to my Mother's card, he realized what must've happened and called me so we could exchange the cards back.

So - before getting to the Studios, my wife and I headed back to the Magic Kingdom. My wife went in to exchange the cards with my Dad. When security saw them exchanging cards they immediately went to check it out and make sure everything was on the up and up.

The problem was, my card was now linked to my Dad's fingerprint. I needed to show ID at the Studio's and the Cast Member's told me would again resync my card with my fingerprint. Unfortunately, it didn't take. Later when we park hopped to Epcot, the card failed again and we needed to repeat the drill with another ID check and again resyncing my fingerprint to the card.

The point is, the fingerprint scanners are pretty accurate and while we weren't denied access, both my Father and I needed to provide ID to make sure we were who our cards said we were (me twice).