I like this question! And I needed more than 500 characters to answer. 
I wouldn’t compare Nextcloud and Sandstorm, but I totally get it! And it’s odd we don’t see more comparisons, but I think the difference may be the approach each ecosystem has to apps.
In Nextcloud, apps are built to the conventions of the core system. They are little subsystems that allow new features, and/or in many case outside integrations and libraries, to interact with the core systems, like users and permissions, things that are fundamental to how Nextcloud works.
Sandstorm provides containers to run a variety of web software. There is so much web software! And there are a bunch of ways web apps express themselves. Sandstorm makes that both manageable and sane for one or a few people to provide relatively instant access to just so much web software.
I prefer Nextcloud, but only because I also happen to like how the core system works there. If you aren’t into it, it won’t make much sense. And if you are looking for a bunch of stuff that isn’t file sharing and organizing, Nextcloud may be a bit overkill.
That said, it has some solid apps that raise to the top that make it a really powerful groupware solution. I have personally used:
- file sharing/storage/uploading
- calendaring/contacts with webdav connecting every device ever
- text and video chat (though not from a mobile device, I have it on my todo)
- password vault
The Nextcloud app store web interface whatever is trash. I have nothing but love for those folks, and I aint doing anything better, but the app store is a sore spot for sure, and to know what is good you have pay closer attention, I think. Kinda depend on what the core team supports. It’s like how if a WordPress plugin has Automattic as a developer, it won’t necessarily thrill you, but you can be 95% sure it won’t bork your site. And the good stuff increases and gets a little more stable, all the time.
Sandstorm is really neat, but there are some quirks about it that I, as a person that habitually self-hosts a variety of web software, don’t really have to deal with. For instance, a lot of software is built with Sandstorm in mind, if it wants to expose user information is a useful way to the app inside the container.
Now, if I were in a position where I had to support a bunch of web apps for a small team, or just want a sandbox to play with (get it?), and didn’t already invest in a vast infrastructure of cloud services and VPSes, then yeah, I’d reach for it.
That said, I haven’t kept up with Sandstorm since Sandstorm is returning to its community roots. Maybe someone wants to discover more about it!
As for other groupware solutions, I haven’t had to scan the landscape for a while, and Nextcloud gets the job done. I kinda feel that if the thing that outclasses Nextcloud happens, it will pop up on my radar. But if you think otherwise, let me know!
I recommend Nextcloud to lots of folks, because it is also federated! If we get lots of teams and families and clubs and pubunixi and phr34ks using ActvityPub, it just adds to the network, giving us strength and teaching folks we can expect better systems, and not a solution from Google or Microsoft or Apple or Adobe or Amazon. Federation: The Win!
Also, and gah, I hate to say this because they want me to, but it really is a snap to install, via snapd. @tim and I were trying out new features, and I spun one up in a few minutes. It’s pretty dope!