HTMLUnitDriver is fastest (50% faster than Firefox), lightweight, platform independent. However, we don't use it. We always prefer to use Firefox, Chrome, IE but not HTMLUnitDriver.
Do you know why??
Here is the answer:
HTML Unit Browser is a head less browser (you can say browser without User Interface) i.e it doesn't open any window, as it runs on the background and interacts with HTML. So, you have to see results in console. Moreover, you won't be able to take screenshots.
Even you couldn't ask others to search entire console box to find out errors.