By default anyone at telegram can read your messages. By default, nobody at Facebook can read your WhatsApp messages. Telegram is a privacy downgrade here.
Facebook could push an update tomorrow that subtly modifies the RNG in a very difficult to detect way that allows them to easily brute force all messages.
No published source code or alternative implementations so it may take months of RE to notice if it is ever noticed at all.
If only a single party with no accountability controls your encryption keys with binaries they compiled from code they wrote that no one else can practically review with an undocumented protocol they can silently change or backdoor at any time, it is not end to end encryption. It is marketing.
I hate and refuse to use Telegram like all centralized chat services and feel it is hostile to freedom, privacy, and security but unlike Signal, WhatsApp, iMessage and others Telegram at -least- lets independent third parties like F-droid compile and distribute binaries offering accountability for their E2E claims.
Another point is to compartmentalize information shared.
Id rather share my whole profile in parts to 100 different companies who all arent allowed to share it with each other, than all of it to one company.