Don't get me wrong, I use Telegram every day despite their flaws. Their data security guarantees may be terrible, but their user experience is still outstanding. It's a testament to how good a chat ecosystem can be if you don't rely on Electron.
I have no reason to distrust the Telegram team, but the data model is simply not designed to keep messages secret. It's Telegram's biggest shortcoming, in my opinion. Things like channels don't need encryption, but group chats can use some WhatsApp-style crypto.
WhatsApp's UX (basically: trust all E2E keys, and show a little notification if the keys changed) makes E2E available to the common smartphone user without the hassle of say Matrix's manual verification. For those with a more security-oriented mindset, there's always the ability to show encryption status changes so you could investigate.
I'll probably be using Telegram for a few years despite its shortcomings. It feels a bit iffy to talk about sensitive personal matters through Telegram, but I know it'll be a while before I can convince any large group of friends to move to something more secure.
For now I'm bridging my Telegram accounts through Matrix, so it's not like I'd benefit much from the added security anyway.