It depends. By default, it uses a weaker encryption than WhatsApp. You can turn on e2e encryption, but not in group chats.

On the other hand, it has multiple FOSS clients, will work on pretty much any platform, and has a great UI.

If you want a fairly secure chat app that your grandparents can use, then Telegram is perfect. If you're sending highly confidential stuff, then no.

It's also suitable for project groups, because of the better tools (and moderation bots) available to the mods.