I will be using only bitcoin only as an example because it is the truly decentralized cryptocurrency.
Problem like what? Decentralized means greater responsibility because you have your coin yourself, not with anyone but you. You will be the one to know how to protect it. Protecting it is not difficult but require knowledge about how to avoid mistakes and scammers that can result to lost of bitcoin.
2. Miners service are decentralize.
Do you want it to be centralized in a way the mining fee will be gotten by only centralized small number of people that can manipulate the blockchain?
A certain user could mistakenly send high amount in the transaction without checking it and will be requested for a refund.
There are some transactions you will make with fiat that your fiat is gone forever. Just know the rules to send to right address.
4. Complete decentralize platform may gone wrong.
Bitcoin and it's blockchain has never gone wrong unlike centralized means which are faulty by nature.
5. Abusers are free to do scamming in their own decentralized way.
More scam in fiat than in crypto for now to let you know how faulty centralized platforms are.