Well, maybe I used to work with more complex data and data-flows but it seems to me that sooner or later to run into database limitations.
For example 1 simultaneous write is quite a severe catch. Also highload app (imagine you need to scale 10 times with the growth of steemit users) demands data separation into hot and cold sets. Indexes and recent partitions on SSD and old data HDD plus you can decompose most of tables into cold-hot rows. So you really need to write new sets into db and manage/process another chunks simultaneously.
And lets not forget completely about replication/failover... It would be sad if you had lost your data and now end up with 3 days downtime in order to re-process all blockchain data in single thread.
I know full-sale DB engine is an investment of resources but calculated and with strong roi prospects.
Just my 2 satoshis.
Cheers!