Reagan had been tested and tempered during his terms as Governor of California. Unfortunately, Trump has had no such experience, no such testing. He has never been in a position of service to anyone.
Domestically, he can be limited by the other branches of government and by our constitution.
But internationally and militarily, he would be a frightening risk as commander-in-chief of our armed forces. He has clear, documented tendencies to anger easily, to attack those who don't agree, and to fail to follow the advice of others. He is a loose cannon, and whatever good he might bring to the office, the huge risk of him as commander of our forces overwhelms all other considerations.