They're extremely versatile with loads of first & third party apps, protocols, integrations, and possible connections.
You can use the cloud as a backup or hybrid, or even stick another NAS elsewhere and make your own cloud of sorts. I don't have experience with load balancing to different sites but it's probably possible.
The backup features are tough to beat cost and function-wise, especially for m.365, g.workspace and hypervisor/vm.
They connect to both enterprise and personal cloud stuff (azure, OneDrive, Google drive, etc.) They also provide iscsi, replication and so much more.
And yes there are mail and calendar apps provided by Synology. You can also find and install third party apps for it, as well as use docker to host anything.
Oh and some of the apps like "photos" can replace, for example, Google photos both the service and phone app. It will do face recognition and automatic scene recognition all on-device.
I've probably only barely touched on what they can do, all without a steep learning curve.
They're the bee's knees.
Zoho Workspace replaces Google mail/drive/docs/calendar/sso/tasks/meet/etc.
Most concise overview I could find is their pricing matrix page: