Worth quoting at length and reading in its entirety, Phat Data from Bill de hÓra:
Good luck explaining to data professionals and system architects that centralised relational databases are not the right place to start anymore. They work really well. There is a ridiculous amount of infrastructure and expertise invested in RDBMSes. Billions of dollars. Man-decades. Think of what you get - data integrity, query support, ORM, ACID, well understood replication and redundancy models, deep engineering knowledge. Heads nodding in agreement at your system design. Websites in 15mins on high productivity frameworks. Java Enterprise Edition. You'd seem to be crazy to give that up for map-reduce jobs, tuple models, and tablestores that can't even do joins, never mind there's zero support for object mapping or constraints. It's no small ask to let go of these features. Psychologically, the really hard part seems to be giving up on consistency. The idea of inconsistent data by design is odd-sounding thing to be pitching, no matter how many records you're talking about. You're in danger of sounding irresponsible or idiotic.