The primary challenge for systems architects is the potentially massive throughput required which could exceed a million transactions per second. NoSQL databases can handle the data velocity but have the disadvantages associated with a lack of SQL access, no transaction support, and eventual consistency. Finally, they don't support flexible join operations, and analytic query options are limited or non-existent. This means you can quickly retrieve a key-value pair for an event, but analyzing the data is a severe challenge.
However, it doesn't stop there.