Feel free to correct or elaborate here, but -

Signal is a centralized service and likes do do things their way (from what I hear, they're not as open to external contribution even though their app is open source), so there is some risk there. But they've had data requests and had very little to give. Data is encrypted, but only passed through their services, they dont keep your data and you need to keep your own backups.

https://signal.org/bigbrother/cd-california-grand-jury/

Telegram, on the other hand - they are also a centralized service with encryption, but they own the data and keys to the encryption. So if asked, they will have to comply and unencrypt the data that is stored.