A problem with shared-everything clusters like RAC is that they tend to scale poorly under write-heavy loads. Other problems would be the significant complexity

A problem with shared-everything clusters like RAC is that they tend to scale poorly under write-heavy loads. Other problems would be the significant complexity, low predictability (esp. in terms of latency) and ofcourse the oracle tax. It boils down to the old question of right-tool-for-the-job and a RAC cluster is not the right tool for most webapp scenarios.