If you're using your own WfVariable to store comments, I don't know that you can retrieve them with WC's Query Builder. As Dmitry pointed out, they are stored as a BLOB.
Assuming you actually are using WfVariables, keep in mind that they are tied to the WfActivity, not the WorkItem. As Dave pointed out, you can get the WfVariable in a WfActivity transition with "self.getObject().getContext().getVariable."
self.getObject would return a WfActivity (WfAssignedActivity)
WfActivity.getContext returns the variable holder (BLOB).
This makes me wonder if you could create a Custom Data utility to retrieve these things, and use a table view instead of Query Builder. I don't use the Query Builder much, so I'm not sure how customizable it is.
Assuming you're using the OOTB comment functionality, that's stored in different places.
If your WfAssignedActivityTemplate requires a signature, you could use LifeCycleSignature.COMMENTS.
LifeCycleSignature is tied to your pbo via LifeCycleSignature.SIGNED_OBJECT.
If you have the correct auditing enabled on your template, you can retrieve WfVotingEventAudits. These are tied to individual iterations of your pbo. You can find them with a combination of WfVotingEventAudit.activity/process.pbo. And they have a WfVotingEventAudit.USER_COMMENT and WfVotingEventAudit.USER_REF available.
Good luck.