Telegram doesn't even have E2E encryption for all chats lmao. But even for those that are, the algorithm they use isn't the best, it's had several vulnerabilities in the past.
(They used to use SHA-1, which was laughably obsolete, they updated this a little while ago, but since then vulnerabilities have still been found)
Your messages get stored in the cloud and their server infrastructure is closed source.
After the Ukraine war began, Telegram also agreed to send user data to Russian authorities when requested for it.
There's also the fact that prior to the previous elections, Telegram deleted pro-opposition channels, defending it by saying "if we didn't we'd be blocked in Russia again" - tough shit. Then Russia can block it ffs