The big problem I have with the Senate is that it's too powerful. Plenty of countries have upper houses that aren't explicitly democratic, but they are constitutionally subordinate to the lower house. The American Senate however, despite being blatantly anti-democratic is even more powerful and senior to the (democratic) House of Reps. The power
People who think the Senate isn’t representative enough clearly don’t understand the purpose of the Senate to begin with.
On the contrary, most are very aware of the purpose of the Senate, just disagree with that purpose. This is a very weak argument I've seen trotted out to defend the institution, but it could easily also be used to argue in favour of, say, the Estates General or the old House of Lords.