One thing that often disrupts momentum and really shouldn't is competitors.
Ideas by themselves are not worth anything, only executing well is what creates value.
Great execution towards a terrible idea will get you nowhere.
AirBnB happened because Brian Chesky couldn't pay his rent, but did have some space.
Most startups are not nearly focussed enough. They work hard...maybe, but they don't work hard on the right things.
A third advantage of mission oriented companies, is that people outside the company are more willing to help you.