I've been using this app for a while now and trying to get in touch with the devs regarding some basic privacy concerns. Essentially - all our data is stored on their servers (unless in the case of "Secret Chats" - which is not the default / passive option). So what happens if you OR the person you're chatting to gets a new number, loses their existing phone, or just deletes Telegram (as you would WhatsApp)? It seems all our data, chatlogs and files (pictures, videos, media, etc) stays on Telegram's servers indefinitely.
Given that this app is so focused on privacy and security, this seems to be a questionmark.
It's exactly the same WhatsApp. Just that whatsapp has no open API and no secure Chats.