A research done by a team originating from a number of German universities has been published. The findings are quite scary for whoever runs a full node of blockchain, including Bitcoin.
The problem may arise from the fact that each transactions record can contain small amount of metadata, like subject or description. The nature of distributed public blockchain makes it impossible to police content submitted to blockchain and therefore the system is open to abuse.
This openness can lead to pollution with illegal and criminal content. Researchers have found several instances of illegal pornographic content and collections of links to child pornography websites.
Bitcoin full node operators and anyone who hosts full node may find themselves in a difficult situation as they may be held responsible for possession of illegal or criminal material. There's a potential risk of prosecution.
Bitcoin's immutability
Bitcoin's immutability assures no transaction or content can be simply removed when written and other blocks have been added to the blockchain. The research concludes that this feature pose a severe risk to the Bitcoin blockchain in the future and may also impact other cryptocurrencies with similar properties.
More details about the research can be found here:
https://www.researchgate.net/publication/321192957_A_Quantitative_Analysis_of_the_Impact_of_Arbitrary_Blockchain_Content_on_Bitcoin