I assume you're talking exclusively about cryptocurrencies, since you added that tag to your question. 

I would say that one perceived downside of decentralization is a lack of a central organization behind the cryptocurrency. Meaning that it could be more difficult to properly organize marketing & user support for that cryptocurrency. 

There is not one single company people can go to when they are experiencing, that's confusing for people. I can almost guarantee that a lot of novice Bitcoin users must have been in panic mode when they made a mistake with a transaction and there was no official entity to ask support.