Why is the necessary and proper clause often a source of controversy and political disagreement?

These two words are vague, at best. What might be considered "necessary" for one political viewpoint may be seen as "frivolous" by another. In addition, "proper" might be seen as "governmental overreach" by some people and groups. Much of the time, this has been claimed by states to create legislation that might be considered out of the purview of the federal arm of the government.