They aren't very reliable. The most reliable devices, the ones at top security agencies, are very expensive.
Reliable in what sense?
Cuz I find that a little hard to believe since I use a fingerprint scanner at my gym (a 24 Hour Fitness in Riverside County) to check in every day.
How it works is I punch my phone number into one of about 4 keypads at the front counter and then hold my forefinger over the scanner next to the keypad for about 10 seconds. When the light turns green, I enter the gym.
BTW, I've had no issues using the fingerprint scanner for entry at either the Eastvale or Irvine 24 Fitness gyms either.