Most notably the fact that it is a black box for the user what happens under the hood. Too much trust required. For me to ever consider something like this, it would have to be open source.
If I wanted to share private information, I would likely use OnionShare which is open source and rather easy to use.
If you want an open source tool, then you can use this free Diffie-Hellman exchange tool I made three years ago. It is a webpage that can be run locally as `file://` to protect from Javascript backdoors. Simply use "Save As (Webpage, Complete)" in the browser and save it. It's designed to be run locally. The code is simple to review fully, since it merely wraps browser's native libraries.
https://borisreitman.com/privacy.htmlI have made Crosspass because as simple as that tool is, it is still too difficult for non-techies.