If by secure you mean the messages cannot be intercepted, yes Signal is theoretically secure as the process of communication is encrypted. On the other hand, Signal can be blocked as it is not truly distributed and requires a Signal server. Very simply put, blocking the Signal server disables the Signal network. That's why Signal is asking someone to setup a proxy server on their hardware that can then allow certain users to connect again to their Signal servers, via the proxy.

Note that these proxies too can be identified and blocked.

Worse, their IP address could be used to identify you in any investigation. This was the legal risk I was pointing out as you do not know how Signal will use your infrastructure (in ways that may make you liable for something in your own country) or how other countries may become hostile towards you for allowing Signal to circumvent their laws.