I have never tried online therapy but had extensive email interactions with my Ts between sessions. I agree that it leaves a lot of room for error and misinterpretation, especially if the people interacting online have quite different thinking and emotional styles. There is just one channel of perception, less reality check, and easy to infuse it with all sorts of biased meaning. Sometimes, for some people, it is easier to write things without direct exposure though, so that can help.