Other than Washington and Eisenhower, generals have tended to make lousy presidents. You can expect obedience when leading an army, but when you are leading civilians, you will never get total obedience. Washington and Eisenhower were good politicians. Grant, Taylor and Harrison were less than great. Garfield had a lot of talent, but he was assassinated before he could do much of anything.
MacArthur would probably have been a wretched president and certainly a one term one.