Unity is hands down awesome for porting. You do have to buy a different license for ios and android compared to desktop development, but the costs there are not extreme. 99% of the code just works the same across the platforms.
Where there are differences are in controls, mainly. You have to code for recognizing gestures, touches, and swipes rather than mouse clicks and keyboard button presses. This is supposed to be more straightforward than usual with unity, but just from a design standpoint this is something of a challenge to get right.
Other things are less obvious that you might have to change. These are not related to code per se, but might make you have to have substantial code changes or even make porting impossible:
1. Your game has to be able to run on the CPUs in question. That's likely fine, but not a given.
2. Your game has to be able to run with the amount of RAM in question. This can be a big barrier for a game like ai war, which uses a ton of ram for gamestate.
3. Your game can't use tooltips, because there's no concept of mouse hover when you're talking about your finger. I mean, you could code that but it would not feel natural. As you know with Arcen titles, that's a big barrier: for your friend perhaps it would not be.
4. The pace of the game, and the difficulty, has to be rebalanced if it is a realtime game, because people cannot react so fast on a phone or tablet.
5. You have to make sure that your game supports all the resolutions of all the various hardware you want to support. This can be very tricky since the screens are so small. In general you may have to really rethink the graphics for a 2d game, depending. Something quite legible on a computer monitor can be incredibly tiny on a phone.
6. You have to learn how the App Store submission processes works, figure out how to support players on that platform, and in general learn how to promote your game in that market. A very different proposition from pc.
Obviously I know you weren't asking why we didn't port yet, but actually you kind of were. Most of our reasons have nothing to do with just direct code. If we wanted a bad port, we could just put in rudimentary motion controls, compile, and submit. That would take a couple of hours max. Doing a great port takes a lot of design work and related hassle.
Which is not to say don't do a port: but that's why I have not. My feeling is that unity makes it so that your hassles are as small as they possibly can be, so it's the best to use.
