The major benefit of comparative reasoning is that it allows us to have some sort of an objective standard against which to compare something that we are trying to analyze. The major risk is that we might choose the wrong thing to compare it with.
Let us look, for example, at the state of our economy today. If we try to analyze it in a vacuum, it is hard to know if we are well-off or not. So we can use comparative reasoning. We can look at past times in history and compare our current economy to the economies of those periods. We can say, for example, that the US economy is weak today because it does not dominate the world the way it did in the 1950s or 1960s. This helps us because it compares the way things are now to the way they once were. But this also shows the risk in this kind of reasoning. What if we pick the "wrong" time to compare our economy with? The 1950s and '60s were a time when the rest of the world was recovering from WWII. Can we really expect that we will dominate the world the way we did then?
Comparative reasoning is good because it gives us standards against which to compare things. But it can be bad if we pick the wrong standards and therefore draw unwarranted conclusions.