The Blockchain Trilemma
The Blockchain Trilemma refers to the existing problem in blockchains where any improvement to scaling, decentralisation or security cannot be made without a detrimental impact on at least one of the other two. Many blockchains have tried to solve the trilemma, only to improve in one area but lose out in another.
This is currently one of the biggest issues with blockchain technology, and cited by many as the reason why blockchains have not replaced existing systems like Visa and SWIFT for payments.