"We discovered early on with cluster that we would have to redesign our application. Our DB was highly relational. Almost no data could be put on the site without data from other tables. We used a lot of joins. We learned (later) that joins in the cluster are not a good idea. Neither are sub-selects."
To be fair, he did then say: "So, we wrote some proof of concept scripts for our application. We were very happy. Very few issues were found. Nothing anywhere near show stopping."
I'm just not too keen on working around those little things called foreign keys, joins and such.