TL;DR Does Telegram really secure?

Some people say Telegram is the most secure and safe messenger, some say it's not. If you're familiar with it you may know from news that Telegram did not gave its clients' info to government, you may have heard that Telegram's encryption is not the best one, BUT my question is does it store peoples' private chats' keys? Actually it does with normal chats because if you reinstall Telegram you can easily get normal chats' messages. Also my friend said that any application in mobile stores like App Store sign a agreement with stores owner company that if some points met, the application owner have to share info of its clients. So dear friend what do you think, should I continue using Telegram)?

P.S. sorry for my not the best English