In MyCase, all comment streams are tied to a particular item (document, calendar event, etc.). Who can see the comment stream depends entirely on the sharing settings for the particular item.
If the item is shared with someone (client or firm user), that person will be able to view the comment stream for the item!
When you are responding to comments from your Comment Inbox, you can easily check to see everyone that the comment stream is visible to. Click the Show More button under the Comments Shared With label (pictured below):
Comments can also be viewed and responded to from the details page of individual items. For example, when I open a document details page for a file in the system, I can view the comment stream for that item. You can then check to see who the item is being shared with, therefore letting you determine who can view the comment stream.
The example below shows an item details page with the Sharing tab selected. The column on the right shows you who can view the item, and who the item is hidden from.