Outlook and the files it generates present several challenges for SpiderOak or any backup solution. The first is that Outlook needs to be closed for its files to be usable for recovery, but Outlook users understandably tend to keep the application open. That means that although SpiderOak and other backup tools can accurately back up Outlook files, in most cases those files would be useless as recovery media. This is an Outlook file format issue, not a SpiderOak issue.
The second challenge is that Outlook files are large and frequently changing, so much so that any always-vigilant backup solution such as SpiderOak can get stuck in a loop of constantly detecting and uploading changes. When this happens, SpiderOak and other similar tools aren't able to perform their other functions.
A third challenge is that Outlook files do not deduplicate well. Users backing up Outlook files find that their space usage rapidly increases with every change, however trivial.
For these reasons, both we and Microsoft recommend that Outlook files not be backed up directly. Microsoft provides an add-in for Outlook that will make backup copies of its file at regular intervals. You can then have SpiderOak back up these copies instead of the original and avoid the problems mentioned above. You can obtain the Personal Folders Backup add-in for Outlook directly from Microsoft.
For further information about managing Outlook's files, see Microsoft's Knowledge Base article on the subject.