DARPA looked into the Bitcoin and Ethereum blockchain implementations, and has documented some weaknesses, like it would only take 4 bad actors to take control of Bitcoin due to the low number of validators currently.
This is no klaxon warning about the failure of cryptocurrencies; this is just documenting what happens to distributed networks when everyone on the network doesn't keep their software up to date. These tools rely on their users following some basic rules - like updating their versions - that, when abandoned, create weaknesses for anybody.
In other words; when you design a new network, the same old networking problems still exist, PLUS the new ones you inadvertently hard-coded into your design.