The strict regulations surrounding the Animagus transformation do not stem from any exceptionally lethal power inherent to the magic itself. On the contrary, the transformation holds little offensive might.
Just as depicted in the original story, the wolf form assumed by Sirius was essentially brutalized and outmatched by Professor Lupin's uncontrollable werewolf state. While Sirius may have recognized who he was facing and held back to avoid hurting Professor Lupin, the incident clearly demonstrates that the animals into which Animagi transform possess no formidable combat strength.
Given this limitation, why is the magic subjected to such stringent control?
Chapter 401