The problem is this: If you're gunning for a Science Victory, you're going to be ahead in science. But if you're ahead in science that also means you're going to be ahead in military tech. So anytime I go for a science victory I'm sitting there thinking: "you know, if I had more cities, I could get even more science. Let's upgrade my existing army and conquer a neighbor or two, and THEN I will go for a science victory." Next thing you know, I'm on my way to Domination victory. As a result I've NEVER won a Science victory in Civ VI.