In case of US the president is elected separately from the legislature.
Step-by-step explanation:
As the president is elected separately he can be of different party and can have different ideologies. The different branches in the US government has been assigned with different powers. The conflict that takes place is due to design. If the president is not good , he has the ability to make changes in the Government as well as war can also start.
On the other side the prime Minister is the head of his or her party and policies made by him will never conflict with the party's opinion. If the government is considered unpopular a no confidence vote occurs and again an election is held.