Using SQL Server Express in real projects

At low load — has. But note that the express edition is artificially limited to a maximum configuration of servers (such as not to use more than one core, any number of RAM, etc.)

If the success (i.e. the increased burden) project you expect to buy (or rent) the "normal" version, then the meaning is, if not, it is better to steer what they can afford, so it was not necessary then to modify the project to use not so expensive, but more functional DBMS