I think Unity is overkill for a game like this. That is why I wonder why Something Unlimited is done in Unity - it would work perfectly well as a Ren'py game. A dev should choose the engine that is best suited for their game from the onset. Changing engines in mid development can lead to the game slipping into development hell and being needlessly delayed - Duke Nukem Forever case in point. Or worse the game never being finished.
While Unity may be overkill, Ren'Py isn't really made with games with these many complicated stats and relations in mind either. Is it capable of doing so? Obviously, we see it in this game. But doing all this stuff may create way more headaches than a "stronger" engine.
Also, regardless of objective qualities, different programmers just may prefer different tools, be they programming languages, engines, other software, because for them it just flows better, they have a more intuitive understanding of what to do. E. g. when studying a STEM subject, TeX/LaTeX is objectively better for writing texts than office oriented word processors. Yet, when given the choice, many people will still use Word or similar because they feel more at home with it.