in my own opinion, a huge disadvantage of an altcoin is when it's crypto project isn't that having high in demands or they're not effective enough to meet the community gap between people and services.
there are too many altcoins today that their price values aren't pumping because of their crypto projects that were not that effective and people didn't see them worth of investments.
Bitcoin is a peer-to-peer cryptocurrency but when there's another variety of altcoin that are almost the same with Bitcoin then I don't think it has sense or would have worth of investments.