It’s obvious.

Telegram was banned in Russia for a short time and for some inexplicit reason unbanned. Russia has made no attempt since then to ban them. The founder of Durov explained that they managed to force Russia to unban them becauye they kept buyint changing IPv4 addresses to evade the ban. But everyone who knows the technology knows this is a stupid reason and completely unbelievable. IP addresses are very expensive and the connections in the app need to change to the new IP. At the rate that Russia and everyone else is able to block IPs (thousands per second) you simply cannot implement such a strategy. What Telegram is saying is dumb propaganda for the gullible.

Telegram insists on not having end-to-end encryption for default chats and group chats and profile and contact data. They use E2EE for Secret Chats but they use MTProto, which is a custom key exchange and encryption algorithm that is doing some fairly questionable stuff according to leading experts in the field. Telegram being insecure is no problem in itself. But they explicitly advertise it as being secure and private and mislead the users to believe no government can spy on them.

They are based in the UAE with their main office being empty and essentially only a mailbox. Nobody actually knows where they are.

Their servers (with unencrypted chat data) are hosted in questionable authoritarian countries all over the world. They do not have any servers in Europe or the US. This is a huge problem since nobody knows what server software they are running and your chats are not encrypted.

If you want to be sure that nobody gets and uses your data use an end-to-end encrypted messenger like Signal. If it’s end-to-end encrypted and open-source then no other questions need to be asked.