I've worked at two of them -- all data, sans pointers to content addressed immutable object storage, is stored in a single database. It worked well for us; it's not rainbows and unicorns wow all of our problems disappeared but we got what we needed with sufficient performance for an app that was not well optimized.

There were challenges like the difficulty of working with a few huge tables that existed since the company's inception and were practically infeasible to do any schema changes on, but that's more a modeling issue. It was also a single point of failure service. If the db goes down so to does everything but constant sense of impending doom it was less of a problem than I thought with ndb.