One neat feature of ido is that, you can config some buffers to be ignored when switching, and in case no match is found, the ignored buffers will be shown and searched automatically.

e..g,
(add-to-list 'ido-ignore-buffers "*")

1. C-x b: buffers like Messages are not shown
2. type * : various ignored buffers matching with * will be shown

This is broken after enabling flx-ido-mode. The ignored buffers are completely gone.