What usually goes wrong
The problems we get called about are remarkably consistent, and almost none of them are exotic.
Nobody owns the admin account
The tenant was set up years ago by a contractor, a nephew, or an employee who has since left. The password is unknown, the recovery email points at a dead address, and the business cannot make a change to its own email without a support case. This is the most common and the most expensive to unpick.
Licences drift out of sync with the payroll
People join and leave. Licences get bought and never released. It is normal to find a business paying for accounts belonging to people who left over a year ago, and equally normal to find a former employee whose mailbox still receives customer enquiries because nobody converted it properly.
Everything is set up as a person
Enquiries land in one owner's personal mailbox rather than a shared address, so when they are on holiday the business cannot see its own leads. Shared mailboxes and aliases exist for exactly this and cost nothing extra on most plans.
Security settings were never revisited
Multi-factor authentication was skipped at setup because it was one more thing, and never came back. That single gap is behind a large share of the small business account compromises we get called about, and it is free to turn on.