It's unclear what you mean about the difficulties of getting data out of SQL Server since it's really no harder or easier than getting data out of any other RDBMS. Scaling SQL Server, just like scaling any other RDBMS, can be challenging since traditional RDBMSs don't distribute well. While some workloads are CPU-limited, that is not true for all workloads so the per-core licensing model of SQL Server (which ~$7k per core for 2016 Enterprise edition, not $16k) is not always prohibitive if your workload is more disk or RAM-intensive. There are points of differentiation between SQL Server, PostgreSQL, and MySQL that might make one better suited to a particular workload than others, but those situations are dependent on the details of a particular workload and something as general as "getting data out" or "on a customer-facing system" aren't specific enough to identify any one of those as more suitable than another.