Telegram also has native, non-Electron apps, for many platforms, could it be an advantage? It also has convenient public chats and "channels" (microblogs). It has support for proxy servers, allows sharing proxy servers and has traffic obfuscation which Signal (I assume) doesn't have.
But for me Telegram and Signal are equally insecure because they don't allow anonymous accounts without a phone number. Using phone number for authentication is insecure because it is very easy to intercept SMS messages, especially if you are a telecom or the government. You better avoid using such messengers and not support them unless you want to provide your ID in future for registration on any website.
The native apps certainly help… Signal’s desktop app is pure afterthought and it shows.
The other thing is that Telegram is just far more feature rich. It feels like what you’d like expect from a modern messaging app.
Perhaps the most unfortunate thing about this all, though, is that third party devs can’t make their own Signal clients to try to improve the situation. If you want to use Signal, you have to take it with all of its warts, and that’s a hard sell.
But for me Telegram and Signal are equally insecure because they don't allow anonymous accounts without a phone number. Using phone number for authentication is insecure because it is very easy to intercept SMS messages, especially if you are a telecom or the government. You better avoid using such messengers and not support them unless you want to provide your ID in future for registration on any website.