Both only reading or only writing is really simple, compared to when simultaneous high load on both reads and writes are needed, especially if at the same time

Both only reading or only writing is really simple, compared to when simultaneous high load on both reads and writes are needed, especially if at the same time as things are flushed to disk in a way that doesn't lose accepted data on power down. That doesn't mean most people can't do well on relational databases as most people simply doesn't need both at the same time.