In comment_entity_info() the comment module is specifying in "entity keys" that its bundle is "node_type", but there is no corresponding column in the comment table. That makes it impossible to query comment entities by bundle without pulling in the node information that each comment is attached to.
GitHub Issue #:
2046
Recent comments
There is also Node Comments Search - this searches the node and all comments (default search is node and first page of comments) as part of the content search. Izzy's module above is a separate...
Search does not include comments
I have created a new module comment_search
Search does not include comments
If there is no response, alternatively, you can use a View with a list of comments, with a filter based on the comment text. I haven't tried it myself, I just got an idea.
Search does not include comments