I'm not familiar with riemann.io but the main differences seem to be that Riemann is clojure-based and would require external storage of some sort. PipelineDB is SQL-based and has integrated storage. It really depends on what kind of event processing you're looking to do.
The main tradeoff with PipelineDB and other stream processing frameworks like Riemann, Storm, Spark Streaming, Samza, and others is mainly flexibility for simplicity. Not all streaming computation lends itself to SQL, but in scenarios where it does continuous SQL queries and a relational database can be simpler. But as with all data processing endeavors, you have to find the right tool for the job.
