IMO the most valid criticisms of Ethereum as it currently stands are:
- Scalability sucks; the blockchain design fundamentally relies on bottlenecks where individual nodes must process every single transaction in the entire network
- PoW is extremely expensive, and furthermore is fundamentally vulnerable to 51% spawn camping attacks with no effective strategy for recovering from one. Selfish mining is profitable starting at 25-33% hashpower, and 51% censorship attacks are definitely profitable.
- Privacy sucks
- It's hard for regular users to hold large amounts of funds without running substantial risks of theft or loss due to theft or loss of their private keys.
- Economics do not encourage good "storage hygiene"; insufficient incentives for clearing storage and insufficient cost for filling it, especially for long periods of time
- Bunch of various marginal technical inefficiencies.
source: https://www.reddit.com/r/ethtrader/comments/6lgf0l/vitalik_drops_the_mic_on_rbtc/dju1y8q/