I understand your skepticism.

Yes, telegram data can be stored in the cloud, but it's not mandatory, it's just a feature to save space on the device. For that purpose I would prefer to use ownCloud client synced with my personal server.

Also, telegram’s servers are protected by the MTProto protocol, while the app offers a secret chat feature that makes it easy to delete messages or schedule a time for them to self-destruct.

It might be a rotten compromise between proprietary software and something that runs on GNU GPL, but for me still it's an alternative worth considering.