Complex sentence: The latter is not to be used as a sentence adverb, it must refer to the subject of the sentence; its misuse produces more deplorable results than "hopefully", and may have furthered disdain for the latter.
Split into simpler sentences: The latter is not to be used as a sentence adverb, they state; it must refer to the subject of the sentence. The misuse of "regretfully" produces worse undesired results than "hopefully", possibly contributing to disdain for the latter.