Bitcoin (rated C+) gets excellent scores for security and widespread adoption. But it is encountering major network bottlenecks, causing delays and high transactions costs. Despite intense ongoing efforts that are achieving some initial success, Bitcoin has no immediate mechanism for promptly upgrading its software code.
Ethereum (B), the second most widely adopted cryptocurrency, benefits from more readily upgradable technology and better speed, despite some bottlenecks.
Novacoin (D) and SaluS (D) are weak in terms of both technological innovation and adoption.
Steem (😎 enjoys a relatively good balance of moderate strength in nearly all the key factors considered along with a social network feature.
You can make the same argument about bitcoin. How is bitcoin decentralized if anyone who can acquire 51% hashing power can control it? And you can't change the hashing algorithm because unless people coordinate that change, they won't agree on the longest chain. 1/2
Heck, you can argue bitcoin is totally useless because unless everyone agrees on the chain validity rules, they can't agree on the longest, valid chain. So often I see anti-Ripple arguments that nobody would accept as valid if aimed at any other system.