The `diffusion.createcomment` Conduit API call was added in D6923 as deprecated (no idea why except, that @epriestley asked it back then). The only concern back then with that API call was that it doesn't support transactions.
Now it does, and works normally: https://secure.phabricator.com/diffusion/P/browse/master/src/applications/diffusion/conduit/DiffusionCreateCommentConduitAPIMethod.php
Can you please undepricate it?
Another useful thing can be adding `identifier` input parameter (optional). This would allow adding comments to commits by their id, e.g. "rABC434346", rather than doing a search first to get PHID of commit and then using it in 2nd API call to add a comment.
I already have a code at hand for that change and can submit it.