In this case a Lumia 950 stocked to the gunnels with music, documents, apps, and so on. I did a manual backup, just to be sure (though these happen weekly anyway, it's an OS function) and then did a factory reset. Signing in with the same Microsoft ID, what exactly would I end up with?
After a few hours of spinning gears, auto-Store installs and more, here's the bottom line, organised by content:
|Start screen||Restored automatically, though some third party application tiles may not come back, so you'll have a few gaps. If you want to fill these, best take a few screenshots of your set-up before you do the reset. Yes, it's all a little trivial, but it only takes a few seconds to re-pin those tiles/apps that are missing, so...|
|Accounts||Email and PIM accounts are restored automatically, though you'll have to 'Fix now' for each in Settings, to re-authenticate, of course. Other app and social accounts depend largely on the apps concerned and their implementation.|
|Files, documents and downloads||
Not restored. This caught me by surprise, for some reason I'd always assumed that stuff in the Documents folder, at least, would be backed up, but it seems not. It's rarely a dealbreaker because in most cases you'll have either downloaded such documents or other files from somewhere else, or you'll have re-uploaded or shared after whatever work you did was complete. But still.... well worth noting. Use the built-in File Explorer app to triple check what you have in Documents and either copy items to microSD or upload them to OneDrive* or Dropbox or similar.
* This is, after all, the whole point of OneDrive, to have a writeable disk in the cloud that's available to all signed-in devices and which is backed up by Microsoft.
|Text messages||These are synced from your Microsoft account from this and other Windows 10 phones.|
|Photos and videos||Not restored as-is, but provided you've had the default OneDrive camera backups running in the past, then thumbnails for all photos & videos will be available in the Photos app and you can edit/re-download if ever needed using Photos/OneDrive, as appropriate.|
|Music||As with files and documents, any local music on the internal storage of the phone doesn't get restored. Now, this music will have come from somewhere, whether side-loaded from a local PC or downloaded from a music service or similar, so getting any local music back is usually trivial, but it's worth noting here at least.|
|Apps||Restored automatically, as in fresh downloads and installs are triggered from the Store. If you had any old, no longer available applications then they won't come back, though it's worth looking in 'My library' in the Store and trying your luck from your past downloads.|
|App settings and content||Restored automatically if the applications concerned support Windows 10's APIs (most UWP apps do). This extends to large data sets, even hundreds of Megabytes in the case of (e.g.) third party mapping apps.|
|Windows 10 Offline Maps||Restored automatically|
|Games||Restored just as for general apps, though very few support their data being restored too. A few of the bigger games have their own sign-in/sync systems in place.|
As mentioned several times in the past, this level of 'restore' is some way between the best in the industry and the worst. On the iPhone, one can typically wipe and restore and everything's back, right down to music tracks, local databases and settings, the works. Mind you, you also don't have a file system or the drag and drop openness of Windows. Going to Android, the state of restore after a reset varies wildly. At its best - typically on the Google Pixel devices - you get about as much back as under Windows 10 Mobile, while on other Android phones, at worst, you get nothing back other than your signed in PIM data.
In short, the only way the grass gets greener in terms of restoring from a recent backup is on the iPhone (or maybe that should be the grass getting 'whiter'?) And going to iOS introduces many other UI and computing compromises that won't suit everyone.
Hopefully I've shown above the only two areas in which you have to be a little careful. As I hinted, personally, any documents of downloads on my internal storage are by definition copies of something somewhere else that I might have used or worked on and then copied out. And as for music, mine's all on microSD and all my Windows phones support this. Thankfully.
What about you? How does the regime and procedure fit in with your experience? Have you ever actually lost anything after a reset and restore?