It's not a "no-no" to use a shotgun mic indoors. The problem isn't the mic, it's the boom-op.
It takes a great deal of skill on the part of the boom-op to avoid getting the hollow, roomy, echoey sound, and a shotgun mic exaggerates the effect. Since most indie projects don't use a skilled boom-op using a hypercardioid mic mitigates the ambient reverb problem problem somewhat.