* Proof of Work is unethically wasteful. Proof of Stake is unethically oligarchical.
* Far too wild of price swings to be used as a currency.
* Inventory tracking with blockchain has the same problem as any other inventory tracking system, that it relies on humans putting in truthful information.
* Smart contracts assume that the hard part of a contract is the enforcement, not the meeting of the minds. Having a human as part of the conflict resolution (i.e. the legal system) is necessary.
* Irreversible transactions are a bug, not a feature, and are what drove the rise of ransomware.
* Negative impact on the GPU market for actually productive uses.
Cryptocurrencies are fantastic for money laundering and paying for illegal services, but I don't think those are exactly good reasons to keep them around.