Secure against what or who? Nothing is completely secure.
Your browser isn't secure, and most Tor users are using browsers over it to visit websites. All an attacker has ever needed to do is compromise your browser.
This is why you should use Tor only with a series of VMs, one with internet and tor access to act as a router and the other which can only access the router vm. This lowers the attack surface to just that of Tor's proxy interface and your VM isolation. A physical implementation of this could be done for increased security.
TAILS is not designed to be run in a VM. You can do it but it's not recommended. Whonix on the other hand is designed with VMs in mind. Qubes running Whonix isn't a half bad solution, though a live CD of TAILS is probably best becuause your system will have relatively few traits that can be uniquely profiled to you. https://panopticlick.eff.org
Secure against what or who? Nothing is completely secure.
Your browser isn't secure, and most Tor users are using browsers over it to visit websites. All an attacker has ever needed to do is compromise your browser.