My company recently switched to MSFT Teams, and the other day I was thinking how difficult it would be to get spam through teams.
By design, only people part of our organization can communicate with each other. There (AFAIK) is no endpoint for my organizational teams account visible to other teams users outside my organization. For spam to occur, someone's account would need to be compromised (which would be found quickly) or a fake account somehow created by the org admin (which seems unlikely).
With email, the IT department is commonly spoofed, or the president, HR, etc. Most people can spot the differences, but with teams they would virtually never get any fraudulent messages.