Oh, where to begin. Telegram is wild. It may not be spyware in the traditional sense, but they've already handed over data to the Indian government, left a telephone number scraping vulnerability open for the Iranian government, and gotten caught with "the most backdoor looking bug" with their unwisely handmade encryption algorithm.
Telegram's backend is proprietary software and they (very similarly to Discord for example) can just decide to read your chats whenever they want. It's even worse then WhatsApp in this sense (at least as long as you trust Facebook that they actually encrypt your chats, again, there is no way to know if it's proprietary software).
Telegram and signal are both central points of failure. Signal can be used with other servers, but the server address is hard coded in the app, so you have to deploy your own app. Matrix servers can keep a channel going even if the channel's home server goes down. The more home servers there are, the more mirrors of public channels there will be.