(FWIW: I prefer Signal.)
Yes: in Telegrams case I might possibly, maybe at some time be taken advantage of.
In WhatsApps case it would surprise everyone greatly if one isn't - sooner or later - take advantage of.
FWIW you can also create multiple groups with the same person(s) so you can keep one "group" chat with your spouse for chores, shopping lists etc and another for photos of the kids, birthday planning, funny stuff etc.
This probably works in most messengers though, but it is a nice hack anyway.
This was all in the news: Facebook lying about not being able to connect WhatsApp and Facebook, then trying to weasel their way out of their previous statements.
For all the problems Telegram has they don't seen to have our raw messages more than GMail has your raw emails.
Yep: unlike WhatsApp and Signal they can, technically produce them. (Edit: I forgot to mention: WhatsApp chat logs gets uploaded unencrypted to American Cloud providers. I have less against the police and Americans than many other here but lets not pretend end-to-end encryption helps for anything when you backup the data unencrypted with everyones favourite villain it seems: NSA)
And yes: like with GMail if Telegram has done exactly what they said and done it properly it would probably take cooperation of at least two sysadmins on different teams and it woumd also probably trigger alarms east and west.
At least that is my understanding.