Web browser platforms are also non-existent with MonoGame. There is something, but I don't think the MonoGame community cares much for those platforms and it'll be a slow process.
Microsoft is opening up .NET. The alternative is Mono, which I believe is developed by Xamarin, which was bought out by Microsoft. Either way, .NET openness will allow MonoGame to access more platforms far easier and faster.
The only differences in cells and cells (The same difference) are texting/calling and buttons. It really boils down to what Nintendo is doing. From mockups the NX gamepad is a cell with sticks inside the touchpad and I'd imagine shoulder buttons. In business terms, cells make the most money being free with advertisements and IAP's. While the 3DS/Vita "cells" are premium priced.
It might also be possible to integrate MonoGame into Source, Unity, Unreal, and other game engines, allowing you access to other platforms. There is an XNA example inside of Unity. But there is not much interest or development being made on those fronts.
And finally, welcome to MonoGame! The powerful XNA code/content with the endless possibilities of the MonoGameTeam/Community, write once, play everywhere, and reach everyone!