As an occasional user: here are a few complaints:

There are too many variants, all only vaguely compatible.

The iOS integration requires enrolling one’s phone in the email provider’s MDM, at least to some extent. This is nice if you’re an admin, but it’s not so nice if you’re a user who uses (in accordance with company policy!) a personal device.

The integration with Mail.app is abysmal. It makes my memories of Eudora seem happy.

Signing in is a real PITA.

The spam classifier is comically poor. I’m honestly surprised that (hundreds of) millions of dollars aren’t lost every year when a (paying, enterprise) customer emails someone at a different business (from the native app!), they reply with an utterly non-spammy reply, and the reply is classified as spam. Seriously, the open source spam classifiers from the early 2000s understand threading — how can Microsoft fail to classify individual replies as not-spam? Google is far better. Fastmail is far better. Everything is far better.

I will give MS some credit: the iOS Outlook app is actually pretty nice.

Outlook is a once-great product that has been left to rot by a Microsoft with little lineage to the great company from the 90s and early 00s that created it.

Outlook debuted Cached Exchange Mode in the early 00s, popularizing "offline first" before it was known as that.

Now: The "new" Outlook can't even show folder unread counts correctly, even when fully online. It seems to only load a small subset of messages locally, only populating folders when you scroll past the point it loaded. (In classic Outlook, this was a setting—I understand loading all mail was not enabled by default—but it could be enabled. No longer.) It sometimes gets stuck where it won't show new mail until restarted. (Gmail has this bug too.) It forgets open mail windows when restarted. It forgets expanded folders in the folder pane when restarted (but only sometimes).

Microsoft removed the ability to show the mail/contacts/calendar navigation bar below the folder pane, and forced it to be shown on its own huge vertical bar, almost all of which is wasted space. For good measure, they did this in classic Outlook as well as "new" Outlook. There was massive backlash to this, and Microsoft plowed forward anyway. On Windows there is/was a registry setting to revert this (but intentionally, no user-facing setting). I have not checked on Mac.

To the sibling comment: Outlook for iOS is indeed great, probably only because it was an acquisition. It is not in Microsoft's DNA to build an app like this themselves anymore.

As an admin: Microsoft seems to redo the Office 365 admin interface every 2-3 years. It is an incomprehensible mess. I am also a Google Workspace admin for the past several years, and theirs is far better, and it's more or less stable over the long term.

Office 365 has been hacked by state actors recently.

I still like the Outlook UI and feature set better than Gmail (despite the "new" Outlook being a major regression), so I begrudgingly stay with Outlook/Exchange because I dislike it less than Google Workspace.

Fastmail does not give signs that they are a relevant company—they created JMAP and basically did nothing with it. Why not make a first-class Windows/Mac client, offline first, with powerful organizational features, like the Outlook of yore? Or at least, contribute to adding first class JMAP support to Thunderbird? This is your sole business.

Especially the point about Fastmail really is a good point! You nailed it with your comment. With JMAP in particular, they have been given huge potential with a huge feature set but have not implemented it (especially not in their own products). Especially Offline Usage...

Microsoft randomly blocks entire subnets of mailservers. If you care at all about independent mail hosting (and you should if you want email to not turn into yet another proprietary walled garden) then you should not use Microsoft's mail products.