Recs.
Updated
SpecsUpdate
Pros
Pro Highly secure
You have control over your data/installation and it uses modern standards like two factor authentication and encryption. There is a security bug bounties program and a strong emphasize on security from the Nextcloud GmbH with a powerful community testing every aspect for vulnerabilities. https://nextcloud.com/secure/
Pro Complete online office suite
Contacts over CardDav; calendar and tasks over CalDav; text documents, spreadsheets and presentations with Collabora Online integration and many more apps can be installed directly from an appstore.
https://apps.nextcloud.com/
Cons
Con Requires technically advanced knowledge
For a casual non-technical user it may be too complicated to use.
Nextcloud service requires the user to run and install the software on a device acting as server.
Although extensive documentation is available, it is much more complex to run and requires a lot more steps to install than say Dropbox or Spideroak.
Con No OOB support for NAS
Synology NAS does support Dropbox, Google drive and other ones like Amazon Cloud, and NextCloud doesn't make it easy to add this support. Yes, it is possible to add a NextCloud client using PHP, but once you add it, the options are really limited, which, using Dropbox, is just a quick and efficient experience for workflow and for any user.