How is it not fit for purpose? You'll wish you never asked! 🤣

I guess it's worth bearing in mind that, AFAIK, organisations' O365 suites are in part bespoke so things that are bad at one company might be just to do with its specific implementation. But this is part of what makes O365 bad: if you need to find out how to get something to work, the on-line help is often useless, because it won't apply to your own company's set up. E.g., menus & buttons might be different.

OneDrive is probably the worst offender. Here are problems that I've noticed, or heard about:

1. General MS problem with characters in file names—i.e., files won't sync them until you've worked out which file needs to be renamed. There's no built in renaming tool, which I imagine is pretty easy to implement. But the bigger problem is that I've been in the situation where I've had to retain and share original documents, for quasi-legal reasons. I can't change anything. The workaround I have to implement is to zip the original file and name it something that OneDrive likes.
2. Many people in my organisation work on projects with people out of the organisation. It is possible, though not easy, to achieve this; but sharing ceases after a few weeks.
3. Apparently, OneDrive has problems with subfolders: they disappear!

I've used several other cloud services which don't suffer from any of these problems.

SharePoint:

1. Sharing is confusing. I'll often receive links to Office documents that don't have the right permissions, or somethings failed. Lots of emails get sent from recipients to sender asking them to fix the permissions so that they can do their jobs.
2. Excel in SharePoint is really poor. Many important desktop functions are missing. Worse, filtering and sorting operates on the SharePoint document, not on the specific user's view of it. This has created problems where one person filters an Excel spreadsheet so that they can process things for their job and this means that another person, with a different role, can't see things that they need to for their job. Some people download the Excel file to work on locally, then edit the SharePoint version, as a workaround; so that defeats the whole point of SharePoint.

Teams

Perhaps not-fit-for-purpose is an exaggeration; but these features are, at least, inconvenient.

1. Often poor quality, video; often with cut outs.
2. You are muted, by default, on joining. This makes sense for big meetings; but it happens even on one-to-one meetings.
3. Excessive power use. My laptop needs to be plugged in to use Teams and it's the only time that the fan kicks in to keep it cool.
4. You can't mark a message as unread & pinning is not salient. So if you read a message that you can't process at the time, it's easy for it to get lost in the swamp.
5. New messages, within a Team, are not indicated at the top level. You need to go into the individual Teams area to see if anyone has contacted you there.
6. You can't use Teams on Safari—I think that this is something to do with the security settings+weird things that Teams want to do.
7. As with OneDrive, using Teams with people out of the organisation is not straightforward.

Outlook

1. As with Teams, new messages that are sent to subfolders are not indicated at the top level. This means that you either need to keep the uppermost folder open, defeating the point of sub-folders; regularly check; or miss emails.
2. The mail rules are useful, but there are some important Boolean operators missing so you often can't get them to work in quite the right way.