Damien Katz (one of my cofounders at Couchbase and therefore has a dog in the fight) wrote this take-down of the Dynamo model, for folks wondering why?
The Dynamo system is a design that treats the probability of a network switch failure as having the same probability of machine failure, and pays the cost with every single read. This is madness. Expensive madness.
http://damienkatz.net/2013/05/dynamo_sure_works_hard.html
TLDR quote:
The Dynamo system is a design that treats the probability of a network switch failure as having the same probability of machine failure, and pays the cost with every single read. This is madness. Expensive madness.