Nextcloud is a cloud-based "file sync and share software". It offers

syncing of OS-based directories (Linux, Windows, macOS) with the Nextcloud server ("Desktop Sync client") [1]
a browser-based desktop (the "Web interface") with various "apps" (including "groupware").
additional functionality via an app store [2]

test instances

apps (basic functionality)

Files: the (synchronized) files on the server
- files can have tags and can be shared
- viewers for videos and images
Photos: organization of images
Activities: a log of changes that happened in the apps -- sometimes links are missing to changes, it is not possible to directly jump to them
Mail
Contacts
Calendars
Deck: Allows (kanban-style) note-taking, hierarchical organization into deck -> board -> stack -> card
- Card: has users, description, attachments, timeline, tags
project: collection of items (files, chats, etc.)

Text: Collaborative document editing using Markdown [3]
Talk: Screensharing, online meetings & web conferencing [4]
Polls: kinda like Doodle [5]
OnlyOffice: browser-based text processing
- " types of supported formats (ODT, DOC, RTF, EPUB, MHT, HTML, HTM, ODS, XLS, CSV, ODP, PPT, DOTX, XLTX, POTX, OTT, OTS, OTP, and PDF-A) are processed with inner conversion to DOCX, XLSX or PPTX" [6]
... many more ...

in its realm it seem to provide a very nice and mature package
it seem suitable as an intra-net solution that allows registered users to interact and work on internal data/files
compared to lightweight collaboration approaches (Wikis, GoogleDoc, HackMD) it feels rather awkward and high overhead (many navigation clicks, slower response, administrative)
there seems to be no "window to the world" for outside viewers/participants
it could be useful to consolidate functionality across users (eg, all have the same Calendar), which is perhaps useful for hierarchically run companies
conclusion: in the context of OSE I don't see a use case how Nextcloud can be leveraged effectively