As I have reported above, it is possible to pay the exact fare difference if your Uber Cash balance is not sufficient. I agree it is much more cumbersome to use than before, plus it takes two rides to actually make use of the Amex credits (credits are moved to Uber Cash on first ride, then you use them via Uber Cash on the second ride). I think it should also be possible to simply top up Uber Cash credits in the app, at which point the Amex credits would be applied, but I have not tried it as my Amex credit for this month is already used up.