I've got 12 cards in my wallet right now. Passbook accepts one of them. I'm unaware of any equivalents that would work with even a majority of them.

To pick a single example, in the UK Boots points cards have a pin in and need to be inserted into a reader.

There are apps, like KeyRing, which I use. The problem is unless the merchant has an optical scanner (many are still on laser) they can't scan your phone. So now you or the cashier has to type in a 12 digit number. If it's the cashier, you have to hand them your phone. It's annoying.

Passbook only supports 2d codes, not 1d codes (UPC-style codes) or mag stripes that you typically find on loyalty cards.

Good luck getting Lufthansa to accept a Passbook version of a United premier card.