It’s somewhat difficult, but also not a simple question. Consider:
* Who in those “countries” is proposing the assassination? The answer to that comes with the limited resources, authority, legality, etc., of the person or people actually organizing the attempt.
* According to the various factions in the prospective assassin “country”, to they really want to cause the assassination? What will it really change, that they do or do not want? What will happen to the reputation and diplomatic future of a nation which is caught attempting such an assassination?
Only after those considerations, does it start to make sense to consider the people and organizations in the target nation, and how well they can detect and resist assassination attempts.
It can be practically hard, too. The USA has about as many resources for assassination as anyone’s ever had, and was openly trying to kill Osama Bin Laden, Saddam Hussein, and 50+ Al Qaeda leaders, for several years, and it took them quite a while to get to them, even after invading Iraq and Afghanistan!
Clearly, assassinations of national leaders has only very rarely happened, and when it has, it’s usually been a very unpopular event with unpredictable consequences, including World War One. So merely on that basis, it should seem apparent that usually very few people think assassinating the leaders of other nations is a good idea.