As much as I hat to say it, but feature wise it's not very close to being a Microsoft 365 alternative.
Don't get me wrong, I'm thankful for Nextcloud and hope they can survive, but one needs to manage expectations when using it...
So far our AIO based installation worked well enough (with PC, Mac and Android clients). But we do not have a lot of files (yet).
After bad experience with incompetent MS support stuff regarding our (not cheap!) Office retail licenses I also had to manage expectations there: a lot of time wasted, they only wanted to move us to the 365 subscription model, could not help. Somehow I'm not convinced that with MS365 we would have a simple well-arranged flexible system (but just be bound forever).
It's basically fine but I can imagine the more advanced features not working so well. It has been on my list for years to add recurring task support to nextcloud tasks but it's a pretty big effort since I'm quite unfamiliar with the stack and there is a decent refactoring needed before the feature can be added.
Hosting files, calendar, and contacts is the primary reason I installed Nextcloud. The other features are just gravy.
It's not the best written, due partly to being written in PHP, but I've not seen anything better out there that you can get for free and you don't have to trust a for-profit entity that's dabbling in AI and violates privacy on the regular.
It was frankly shocking to see such negativity about one of the only projects in that space that is achieving goals.