Throughout the development process we run a variety of tests and go back and forth with you each phase to ensure that everything is working as expected.
Also, right before delivery we do another extensive set of tests to make sure everything is perfect so you don't have to worry.
With experience comes the knowledge of how to do things right.
This allows us to know what the common security pitfalls are and to ensure that our systems do not have them.
Web applications aren't of much use if they only work well for one user :)
Which is why we have worked with a variety of different levels of scaling needs, from code optimization needs, to multiple web servers to fully redundant n-server setups.