Another thing to consider is that you lose significant performance in a few different dimensions if your storage I/O scheduler design is not tightly coupled to your execution scheduler design. While it requires writing more code it also eliminates a bunch of rough edges. This alone is the reason many database-y applications write their own storage engines. For people that do it for a living, writing an excellent custom storage engine isn't that onerous.

RocksDB is a fine choice for applications where performance and scale are not paramount or your hardware is limited. On large servers with hefty workloads, you'll probably want to use something else.