The problem with coin divination is the inherent problems involved with flipping them. They aren’t ‘random enough’ for you to really reliably use them for divination because probability rules interfere too much with what you are asking. Tarot cards or Runes are much more reliable, because they are inherently less prone to failure from accidental ‘user error’.
If you flip a coin, there are two outcomes, heads or tails. The outcome is able to be influenced just by you flipping ‘badly’ or not putting enough spin on the coin or any other number of things that could be attributed to what amounts to ‘user error’.
Pendulums have a similar issue.