I see a lot of comments suggesting FastMail and other email service providers with custom domains. Can someone explain how FastMail differs from Microsoft Offic

I see a lot of comments suggesting FastMail and other email service providers with custom domains. Can someone explain how FastMail differs from Microsoft Office 365 Plans like Business Premium which allows custom domains and exchange because to me Microsoft seems to be the better option with Office suite and OneDrive storage. I'm skeptical thinking whether Microsoft could also ban a paid user's account like this without any prior warning or export option.