Since hate groups often have an unrealistic and drastic view of certain other groups, it's not outlandish to think that hate groups will do something dangerous. While most hate groups may not be able to do any damage on a large scale, the actions of the group are often unpredictable, and could end up seriously injuring others.
Yes I think hate groups threats are often underestimated. I know that different government agencies are always watching these groups to ensure they stay out of trouble, but you can never be to safe or vigilant. I'm sure there are even some groups that the government is not even aware of.
Most Americans -- the vast, vast, majority -- reject hate groups and are aware of their propaganda. We know that they have a right to share their views, but they are in the absolute minority and cannot really take strong action to further their views. They pose a threat but are contained because we have NOT underestimated them. Both citizens and the government are well aware of what these groups do.
Groups identified as 'hate groups' are under constant observation and scrutiny by a variety of government agencies, as well as other organizations and media. Investigations and reports are regularly issued and steps taken to interfere or hinder such groups. Despite this amount of attention, such groups' actual illegal activities are negligible to non-existant when compared with other groups such as criminal gangs or foreign terrorist groups.