Currently, it is difficult to use issues/bulk_change to add a comment to multiple issues at once. Just passing issues and comment doesn’t work – produces an error stating an action is required. (This is not how the documentation reads – see Web_api/api/issues/bulk_change description confusing) But adding the type or severity option works, but only sometimes – if the type and/or severity are the same as before, the command is ignored and the comment is not added. But if something changes then the comment is added. So if you add to a set of issues of mixed type, then sometimes the comment is added and sometimes it’s not. So the command isn’t even atomic and one has to go figure out whether or not the comment was added to each issue.
And if comment is included then the update should occur even if, say, the severity setting is the same as what the issue already has for severity.