To be more direct in an attempt to answer your question is a way that you will understand. Windows Phone is designed to be stable. It is locked down to insure its stability. Windows Phone is by nature made in a way that makes the OS more reliable and stable. Stability and the security of the OS are the main features the driving force if you will of Windows Phone.
<snip>
Why do I say instability in a WP is most likely hardware? Because it is the nature and design of WP OS to be stable. Bad code in apps can cause the app to run badly but bad apps can't cause the OS to be unstable that is not the framework of WP.
You know, I very seldom saw any of my WP devices simply reboot for no reason, but apps crash way too much. A stable OS doesn't do a whole lot of good if apps don't work properly. Live Tiles are also a real issue. I realize that poorly written apps have their own problems, but I've had trouble with too many apps and Live Tiles for too many apps to believe it's all the app's fault. MLB finally released an official app, but it didn't work half the time (at least). I had to manually set the time on my phone to a day earlier to get MLB At Bat to work. Sure, maybe locking the OS down so much that you can't email documents (more than one at a time) or join a VPN helps make it stable, but it also seriously affects the usability. Maybe limiting Live Tile updates to every 30 minutes helps battery life, but again, it negatively affects usability.
Is the locked down OS is working against itself? It seems that as a general rule WP apps are of lower quality than their iOS or Android counterparts. I'm not a developer, but I have to wonder if the devs don't have the APIs available that they need to make the apps work as well as is possible on iOS or Android.
In theory, Android by design may be unstable. In real life, on a Nexus device, it is very stable. Like I said earlier, my Nexus 4 was running a month and a half with no issues. When I did reboot it, the reason was that I was doing a hard reset because I was transferring it to my wife. For the most part, instability in Android is either from a poorly written app or the manufacturers' launcher. That is proved by Nexus devices. And by the way, you said you're looking for a 5" device? The Nexus 5 is close, at 4.95".
I'm still hoping that WP makes some more progress. After all, I'm employed by a Microsoft partner, and I have a few Microsoft certifications. I'd love to jump in to the total Microsoft ecosystem with both feet and never look back. Twice in the last year I made the move to WP, but I soon retreated. I even had a 920 at one point. Probably for me the the two biggest strikes against WP were a very poor Facebook app, and no MLB At Bat app. The 2nd one was provided halfway through the baseball season, but by that time I had gotten rid of my 920, and got a cheap 520 just to keep my feet in the water. As it turned out MLB At Bat didn't work half the time anyway.