Ben Thompson of Stratechery puts it well:

This is where Teams thrives: if you fully commit to the Microsoft ecosystem, one app combines your contacts, conversations, phone calls, access to files, 3rd-party applications, in a way that “just works”; I explained my personal experience with Teams in a December 2018 Daily Update:

"Here’s the thing, though: Dropbox absolutely is better than One Drive. Google Apps are better at collaboration than Microsoft’s Office apps. Asana is better than Planner. And, to be very clear, Slack is massively better than Teams at chat. Using all of them together, though, well, it sucks: the user experience that matters for me is not any one app but all of them at once, and for the way I want to work, having everything organized in one single place is simply better (and that’s even with the normal spate of maddening Microsoft UI oddities!). In this Teams is less a chat app than it is a file explorer for the cloud generally, and Stratechery LLC specifically."

This is what Slack — and Silicon Valley, generally — failed to understand about Microsoft’s competitive advantage: the company doesn’t win just because it bundles, or because it has a superior ground game. By virtue of doing everything, even if mediocrely, the company is providing a whole that is greater than the sum of its parts, particularly for the non-tech workers that are in fact most of the market. Slack may have infused its chat client with love, but chatting is a means to an end, and Microsoft often seems like the only enterprise company that understands that.

It's been my experience, too. M365 really actually just works and Teams and Outlook (for the web) are a pleasure to use.