Postgres/MySQL and friends are incredibly good at handling the kind of workloads they were designed for (transactional reads and writes, and perhaps some aggregate queries if you can plan your indexes in advance.) You can scale that quite far, although if you get big enough you will eventually run out of room there (around 10k writes/sec or so.)
However, relational databases are a poor fit for writing logs, for performing analytical queries, and various other purposes that most reasonably large businesses today need. You can try to use your relational DB for them, but you will create real performance issues very fast; and even if you can get the DB to scale up for the load, you'll end up paying way more money that you would from using the right kind of DB for that workload.
However, relational databases are a poor fit for writing logs, for performing analytical queries, and various other purposes that most reasonably large businesses today need. You can try to use your relational DB for them, but you will create real performance issues very fast; and even if you can get the DB to scale up for the load, you'll end up paying way more money that you would from using the right kind of DB for that workload.