It works if you accept the UI compromises that have to be made to have it work on dissimilar devices. You may be willing to accept all of that, but I've grown accustomed to the UI excellence of Windows XP and 7 over the past 16 years, so I expect nothing less than that now, and I can't get that in 10 (and it's gotten worse since 10 first arrived). I assign zero value to the ability to snap a keyboard on a tablet and have it be a "laptop," so that doesn't offset the negatives for me.
The thing is, Windows 10 is already a jumbled hodge-podge of desktop and mobile elements. The desktop itself is all Win32 with its typical common controls, and so are all of the x86 programs that run on it. Many of the system dialogs, however, have had their Win32 versions removed, and they've been replaced with "app" versions. There's little rhyme or reason to it. Right click the Win32 desktop, then select Personalize on the menu. Instead of the Win32 personalize menu you would have gotten with 7 or 8, you get an "app" personalize menu. Hit "themes," and the same old Win32 themes dialog appears.
In time, that Win32 themes dialog will probably end up being an app too, which is going in the wrong direction for the desktop user. Even though these are system dialogs that have multiple entry points, they all fall under the control panel hierarchically, and that, of course, is being systematically dismantled and replaced with the settings app.
The convertible device is an edge case. Most devices are not convertible. Why, then, should that specific use case be the one that defines Windows for all devices? A Jack of all trades is the master of none, and a single UI paradigm that attempts to work on everything only ends up in delivering mediocrity to a wide variety of devices. A purpose-built mobile UI is always going to be superior to one that has to make significant compromises to accommodate devices with very different characteristics, just as a without-compromise desktop UI (like the one found in Windows 7) will likewise always beat one whose very existence is a compromise.
Ultimately, a device needs a UI that is optimized for the manner in which each device is used. Note that this isn't the same thing as requiring a separate OS-- an OS is not a UI. A device that has a small screen and uses touch for pointing needs large UI elements and accommodations for those large UI elements (whether or not it has a keyboard attached); that means a mobile UI. Even a larger tablet with a keyboard attached still uses touch as its pointing device, so a mobile UI is still the best choice. A device that has a large screen and a mouse is much better served by a traditional PC UI.
In terms of the convertible device, though, I'd say that when it's undocked, it would use the mobile UI, and when it's docked (and has a touchpad or mouse as its pointing device), it would use the desktop UI by default, but that it be configurable by the user if that system doesn't work for him. Perhaps the context switch between docked and undocked mode would prove too jarring. In that case, he could select whichever of the other modes he finds more appealing.
Having both modes jumbled together, which is the current Windows 10 design, isn't ideal for any platform.