Hi Bill,
As you are using the item versions, some functionnalities may not be available in the CAML Query. We don't directly retrieve all the data from SharePoint using the CAML query. We need to retrieve the list items using the CAML query, and then the item versions from another Web Services. _UIVersionString is one of the field that we retrieve using the Item Versions Web Services, therefore you won't be able to order by this field in the CAML Query.
If you need to order the items, you will need to add an sqlQuery statement just after your list statement:
<sqlQuery tableName="Ordered Review Comments">
SELECT *
FROM [Review Comments]
ORDER BY _UIVersionString
</sqlQuery>
Don't hesitate to contact us if you need more information.
Regards,
Marien Monnier
Enesys