The biggest usability hurdle so far is that there is no tab-completion. This is why the ex-mode support is not enabled by default and part of the reason why its practical usage is somewhat limited yet - if you don't remember exactly what do you want to invoke, tough beans. Someone shall address this issue in the future.
Also, perhaps wider scale of commands should be implemented in ex-mode. The code is extremely flexible and it is very trivial to make another ex-mode command handler, it's just that no one has done it yet ;-). Also, more actions should be able to take arguments.