I think it depends on the environment you're in. In my social circle, among family, friends, and coworkers, almost all of them believe that socialism is a poverty cult and a terrible idea that will lead to more human suffering, so when I express any socialist opinion, it will be unpopular in my environment. Obviously here on Lemmy, it's the opposite and socialism is popular here.
I mean, yeah, politics, religion and other ideology-driven subjects are more contentious, but apart from these very divisive subjects, there isn't much anyone can be a fan of that is unpopular.
What about food? If you go to Italy and declare that your favourite pizza topping is pineapple, I assure you it will not be received well. If you put chorizo on paella, you would be scorned in Spain... Go to an American police station and declare that doughnuts suck, and you might get shot.
Any topic that people are passionate about, and you are going to see plenty of arguments and see some unpopular opinion pop up.