What's this have that Signal doesn't?
> The Signal Protocol is a non-federated cryptographic protocol that can be used to provide end-to-end encryption for voice calls, video calls,[3] and instant messaging conversations.
> Tox is a peer-to-peer instant-messaging and video-calling protocol that offers end-to-end encryption.
https://en.wikipedia.org/wiki/Tox_(protocol)
(There's a running joke/observation that every time 4chan /g/ tries to produce something, they spend all their time debating logo design instead of working on the product. So for this /g/ project to take off, its logo must be so good as to be beyond reproach.)
Signal is server based. Tox is serverless.
In the case of Signal they also had unusual peculiarities. They have framed themselves as a privacy and security oriented company yet the first thing they require when signing up is a mobile number. And for years they required Google Play Services and were dependent upon Google Cloud Messaging. These sort of things send quite mixed signals, though to Signal's benefit they no longer require Google Play Services.
Finally there is one practical issue with centralized services. They need to make money. Servers cost money. And as the popularity of a platform grows, the amount of money they need to earn also continues to grow. And that's just to exist - not to make a profit. For free services their most valuable resource that can be spun into money is user information. Signal for now has managed to get by on donations and grants. The vast majority of their funding coming from the "Open Technology Fund" [1], which is a US government program. If their funding dries up you risk seeing their ideological views change as a means of self preservation, or the entire service become unusable. This is not a problem with decentralized services.
Ultimately it all comes down to what you'd prefer - centralized or decentralized. Signal is centralized, Tox is decentralized. Ideologically I believe the future of all digital technology ought and eventually will be decentralized, though the time scale of "future" there is quite immense. Nonetheless, I like to do my small bit today and support decentralized tech whenever possible.
[1] - https://en.wikipedia.org/wiki/Open_Whisper_Systems#Funding
And similarly the problem with funding is not one that's paradoxically not solved by money alone because it then raises new questions. In particular why would you put $50 million into a project like this? Is it simple good will and benevolence expecting that money to simply be gone but in exchange society gains a great centralized communication medium? Well, maybe - but it's also possible that there are different motivations. And when this money runs out what happens next?
Ultimately I'm not trying to convince you not to use Signal, I'm explaining where I see problems with Signal. And with all centralized services it's going to come down to trust. If you trust Signal then you might answer the above questions in one way. If you don't trust them, then your answers would probably be quite different. In my opinion no tech company deserves the benefit of the doubt when it comes to trust anymore.
But hey, "Security and privacy"!