I was under the impression that MediaWiki, due to its nature as "open for all Wiki platform", is not tailored towards managing sensitive information.
I found some warnings about this on the MediaWiki FAQ and some user account extensions as:
If you need per-page or partial page access restrictions, you are advised to install an appropriate content management package. MediaWiki was not written to provide per-page access restrictions, and almost all hacks or patches promising to add them will likely have flaws somewhere, which could lead to exposure of confidential data. We are not responsible for anything being leaked, leading to loss of funds or one's job.
Now a consultant of my boss tells him there is no problem with sensitive information at all. I would like to hear if he is right, and I worry too much.
I suppose all problems would go away if we would use separate instances of MediaWiki for every user group with the same rights. Or are there known vulnerabilities?
Edit (some more information on the data):
The data contains information about financial investments (so it is very important for the customer) which should be made available only to key personnel. And this key personnel may well change so the rights have to be revocable. I can't estimate the scope of the data yet, but if the system works well it may be used a lot.
(Remark: This question was asked at Stack Overflow, but since it was considered off-topic there, I ask it again here.)