[MDCore]

I had a problem where it was not possible to include people from different organisations. Once you have more than one external organisation involved, I have to manage how they join and whether "external" or "guest". This makes it difficult or even impossible to set up the channel/chat with the people you want. This is entirely a Teams limitation.

Slack does not have these artificial barriers. You can invite single channel guests, or add them as full-fledged members. It's simple and logical.