Earlier quoted context omitted.
What would you recommend instead?
For security-critical or sensitive situations, auditability should be a requirement. That implies access to source code and capabilty to build it. Decisions like these need to be done from first principles. SharePoint shouldn't even have been a contender here if looked at seriously. Do your own homework.
Vendors can be accountable without providing source code, for example through contracts specifying performance.
I don't know how large Sharepoint's source is, though it has many components and I assume there is quite a bit of code. Auditing the source code of something like Microsoft Office seems almost impossible.
> first principles.
What does that mean in this context?