I would use Tox and any of its clients with caution. At one point in time, your friends would be able to execute arbitrary shell commands on your PC if you were running utox and accepted a file download. Even with large security concerns like this, the lead developer believes Tox and uTox is secure because he reads the code he wrote himself (none the less git history is filled with bug fixes he clearly missed in his reading). This isn't exactly reassuring, especially coming from someone who doesn't have provable past experience in security software.
Edit: I just got banned from their IRC for stating this opinion here.