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The bad part of decentralization is:

1. An organ from the government that is getting bigger and bigger until the structure of the government becomes more complex.

2. Conformity and Balance will be increasingly disrupted between the various interests of local governments.

3. Decentralization of territories that will encourage the emergence of a regional understanding.

4. Because it is used a lot to negotiate, the time needed is increasingly.

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. 

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