if you're going to use Tox, make sure you're routing it through tor. If you communicate with someone that you don't 100% trust, or don't 100% believe their network isn't compromised, you are playing with fire.
Group chat can be DoS'd if you add two bots to a chat, and have them disconnect people from the rest of the group.
Tox is more secure than any other chat client, but be careful with how you use it, and with whom.