Coin apparently aren't a MC/Visa issuer, so they can't claim to issue a valid card or mobile payment device. In addition, merchants are supposed to judge cards by the system logo - and Coin can't have MC or Visa logo if they aren't an issuer, and certainly no card is allowed to have both MC and Visa logo (even if they would technically support both), MC/Visa prohibit that.

On an anecdotal basis, the recommendation given to cashiers when teaching them how to handle cards and detect fraud is not really in-depth but more or less "if you see something suspicious that is not a normal card, call the cops", or something like that.