The query actually holds good in general for every situation in life. It is one of the principles of good time management.

The idea is to tackle hard (and important) problems first. This, if resolved - will pep up your confidence to deal with other not so hard issues. Also, this could have cascading effect on other issues that may get resolved on its own.

I would rather stress on "important" than "hard" issues. If a "hard" problem is not coming in the way of your deliverables (means it is not important) - keep it aside. There is no need to spend a lot of time on it.