As a professional c++ developer, I would have done MotH in Ren'Py.
Well, that's possible is developer (in this case you) has decent command of Python... I have never asked Faerin about his skills in Python, but I know he is very good with C#.
There is nothing in MotH that requires the power of Unity. Ren'Py is simply better (IMO, of course) at VNs than Unity. This game is a sandbox VN. Look at the support burden - lots of people have issues with the resolution, or videos.
RenPy is made fro VNs, as it's name says. But still, this is works fine. Most problems with resolution etc are due to game being optimized for 16:9. As for us Linux and Mac users... well... we are running Windows game on Linux, through compatibillity layers, it's always tricky.
And Ren'Py gives you a free Mac version, and much easier to make Android version.
I guess you mean native. Portability is not issue with Unity. He didn't have time to maintain 4 versions. As for Unity and Android, 90% of games on Android are made with Unity... issue here is, that game is optimized for computer screens. Only portability advantage of RenPy is video support (this concerns Linux users most, as built in Unity player plays only webm and ogg on Linux. This was due licence issues). Anyway, I think Mac versions can appear in some time... sooner than you expect.
PS, about RenPy and Anroid... well, judging from most comments there is tonn of bugs around. There is also problem, depending on architecture, there should be actually two Android versions... and again, game would still be optimized for large screens. Not to mention privacy issues with playing adult game on Google built and tracked OS.
My bottom line would be:
1) RenPy needs decent knowledge of Python. You can see lot of issues with games here...
2) Renpy is indeed better for Visual Novels, but this isn't pure Visual Novel. Faerin will probably explain more about his engine, when talking about new game.
3) Unity actually has decent portability. RenPy, as open-source has better portability, but still there is lot of issues with Android... I would guess similar would go for IOs, due to same limitations of small screens.
4) Most of issues people have here would exist in RenPy. Some would be avoided, but porting this game to RenPy, would be a lot of tedious work... and game already runs fine on Windows, and even on Linux, Mac (I have strong hunch this would work on OpenBSD, FreeBSD etc..)