It makes unconfirmed transactions useless in the classical Bitcoin model. Obviously if you introduce a trusted third party you can fix things, but then you're back to having the disadvantages of centralised trust.

If unconfirmed payments become flaky enough that people stop using them, then a portion of the Bitcoin community will find workarounds like trusted third parties, trusted hardware, whatever and will just struggle one. Other people will look at the new tradeoffs/complexity, and decide that Bitcoin is no longer better for them than banks.