Most (if not all) modern RDBMSs allow for reliable horizontal and vertical scaling. SQL Server (the db you mention) is no slouch when paired with additional CPUs (for vertical linear scaling) and allows for transactionally safe replication to distributed clusters of SQL Servers (for horizontal scaling) in geographically distributed servers. PostgreSQL has this and it's absolutely free.
Where is this meme that RDBMSs do not scale coming from? (I see lots of people saying this as though it is a given. Has there been any published data on this for me to read?).
Where is this meme that RDBMSs do not scale coming from? (I see lots of people saying this as though it is a given. Has there been any published data on this for me to read?).