I trust Slack more than I trust Telegram as the entity responsible for my corporate data and sensitive communications. That alone would be the dealbreaker for

I trust Slack more than I trust Telegram as the entity responsible for my corporate data and sensitive communications. That alone would be the dealbreaker for me. If I was at a tiny startup that was just getting off the ground, I might be more open to using Telegram for the use-case you're describing. But once you're dealing with a real business, several employees and several paying customers with significant revenue implications, I'd switch to a known and trusted entity that has enterprise support (like Slack, Teams, Hangouts, etc).