The first problem is Unity. In my experience, there is quite a lot of fuss when it comes to porting. You have to sift through textures and find which ones should be compressed, gotta make sure all your game can work with small resolutions, etc. While I only have experience getting a Unity game onto Android, I assume there are similar issues when porting to other platforms. Unity is also very inefficient; a simple game like
Space Shooter requires at least 22 MB, which is fairly large for such a small project. For the record, Hearthstone is build in Unity, and it is 2 GB in size (as opposed to the Windows version's 1.34 GB) and is fairly demanding. My phone loses most of its battery power within the course of roughly 8 matches (or about 2 hours), but it can last for a better part of the day when not running Hearthstone.
But say you have a really good phone with 32 GB of internal ROM. Great great. But then you have a much larger problem: an FPS is a terrible phone game.
According to Extra Credits, you can't just port any old game, and this is obvious if you think about it. I mean, it
can be done (Minecraft has probably the best touchscreen interface I've ever seen, but that was a large touchscreen), but it just doesn't work as well. It is probably better to make a spin-off or an entirely different game. (Bethesda's
Fallout is a great example of this.)
It can be done, but it would be a poor business and design choice.