I think the reason some UUs replace units while others don't (if that's the case), is because those units that are replaced would never be used anyway. Like, no one would build a knight if they have a UU that is stronger than the knight, costs the same, and comes at the same tech. Obvious, I know, but that means that not replacing a unit by itself can be worth as little as nothing. So, it makes more sense to compare UUs based on their specific benefits rather than assuming the ones that don't replace units are automatically better. Having a non-replacing UU come a tech earlier or having an early unit with strange penalty (like Korea's UUs in Civ V) might be very situational whereas a UU that replaces a unit but has a very good promotion (like the English bow in Civ V) would be very valuable in more cases.