You may need to be more specific about what questions you are being asked with regard to security of summative assesment. Salespeople often create impressions of software's capabilities that are somewhat out of touch with reality, particularly when the salesperson and/or the customer is non-technical. And Moodle cannot hope to compete against vague or fictional capabilities of other systems.
In general, I believe the security of summative assesments have very little to do with the VLE system itself, and more to do with processes and preparations that surround the test e.g. note items 8 through 13 of this checklist for summative testing in WebCT:
http://www.elearning.canterbury.ac.nz/documents/testtips.shtml