Pretty much a crystal ball exercise. Going into unity means you are trading full control over your engine, for something else. In this instance it seems it's graphics. Only SI can evaluate that side of things. I honestly never expected that if you were looking for a graphics engine your choice would be unity so, there must be more behind it. I suspect easier multi-platform development. They will for sure be more focused in getting everything to work rather than adding new variables into this already complex equation. I'm not sure what was the base language for FM and how easy they can port that into unity. If it's not a direct(ish) port that can be such a monumental task that I'm not sure how it could be completed in the stated timeframes, without some horrendous investment in development. (I wouldn't want to be the one financing such a task.) Now, for the games I know better (wargames, the hard core type) every single one that is done in unity suffers from being sluggish, simplistic, 'light', when compared to their peers done in proprietary c++ or even delphi engines. They do not look any better. That might be down to the fact that the wargame companies that tend to use unity are the ones that either see in it as an advantage for one of two reasons: multi platform or that it simplifies their development environment. No one thinks unity by itself will deliver a better game, they just know it delivers a better game than they would otherwise be able to do. I expect, in the long run, a 'lighter' game that will start to become more and more a portable / console game and less of a PC spreadsheet game. Focusing on graphics possibly also means a long-term aim to have the 3d action become more central to the game. Again, crystal ball.
Edit to say, Every unity game I have also has ginourmous patches. So be ready for that. As an example, just recently, mtg arena had a 7gb patch (pretty much the size of the entire game).