So I experienced a number of scenarios, where almost nothing restored, and others where quite a few settings restored, and almost no apps (as mentioned a lot developers haven't updated to WP10 restore, and probably won't).
Once on my wife's 950, I did a reset (it got all kinds of messed-up, couldn't call, phone was almost hard-hung), and it only restored the background and a couple of other items. My guess is that the backup had gotten overwritten during the corruption, and the restore was the corrupted settings. Ended up starting from scratch, spent a lot of time trying to get everything back to "normal", and then she fiddled for a bit, nobody was very happy about it.
On another occasion, I kept getting the "dim screen" issue, at wake, so I ended up doing a hard-reset. Again, it appeared as if my restore came from a corrupted set of data, as almost nothing restored, apps tried to install, but failed, more or less I had to set up from scratch again.
I've had several hard resets, when running slow-ring, that were varying degrees of successful, I always had to go and re-set-up the power settings, for example, and a few other things, at a minimum.
After AU, I did another hard reset, when things got really bad on my Release Preview ring build, and it took me several hours to get things back, virtually nothing restored. That was probably the beginning of the end for me, the instability, not knowing if a reset was going to take hours to rebuild, etc.
Compare that to my 6P, where I had to do one reset (coming back from Android 7.0 preview to RTM), and it took me about 5 minutes, back to 100% restored.
In the W8M days, the restore process was like my current Android one, it was pretty much 100%, you just got back to exactly where you were. A couple of my hard resets were close, with the exception of some settings, but it seems like they should be moving back to the 8.1 restore-reliability, not away?