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.
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.