I trust Bruce Schneier on this stuff and he advises against telegram: https://www.schneier.com/blog/archives/2016/06/comparing_mes...

Also in the comments on that "article" some people link against a cryptoanalysis of telegram, which can be found here: http://cs.au.dk/~jakjak/master-thesis.pdf

Someone else also mentions that telegram stores message logs on their server, if that's true it is definitely a big no no.