EDIT: Here's what they are saying about Unity:
"We never had a Unity port for Frontiers. Origins is coded in cocos2d, like all the other KR games. By the time Unity became known to us, we gave it a try with the Steam version of Kingdom Rush. This was mid 2013... Frontiers was almost ready by then. Early 2014 we started working on Origins and we used the old engine out, sine the effort of adapting the old games to new platforms wasn't exactly good to us. Origins is the last game that will use our old and loyal cocos2d engine. As we define what our next game will be, we are very much aware that it must be multi-platform."
They wrote Kingdom Rush first in flash, then ported it to iOS, then ported it to their own proprietary mobile engine that's portable across iOS and Android. Earlier this year, they also ported it to Unity.
This blog post is talking about their newest title, Kingdom Rush: Origins, which isn't built on Unity but still on their own proprietary mobile engine.
[0] http://www.ironhidegames.com/forums/viewtopic.php?f=5&t=5950...
I guess they are too exhausted at this point. Even with Unity there are bugs specific to each platform and supporting that takes time.
You might need to adjust some screens so they work better with a mouse instead of with taps, but this game doesn't use multi-touch or anything that can't be done with a mouse so I don't see an issue with that.
You also need to write platform specific IAP and notification code, but again, going to desktop you won't have notifications, and writing platform specific IAP code is not that big of a deal, especially compared to porting your entire game.
Short story is, these guys chose their engine very poorly. They had already experienced cross platform issues in the past, and seems like they just decided to ignore them until it was too late.
I have no sympathy.
In my case I bought one game and was hit by this bug. I contacted developers with strace data and got no response. Then I contacted Unity developers and they actually got back to me and confirmed that it's indeed the case with large XFS partitions (they had to set up that test case to verify it). They produced a custom build of Unity player of the same version with patch backported, and I was able to run the game.
Now, those game developers obviously don't care about supporting such cases, yet they sell the Linux version. I don't appreciate it on one hand, on the other hand may be it's better than not having that version at all. But I can see a point of some being exhausted and trying to avoid bug reports for systems that they can't properly support.
The proper response from a Unity game developer who wants to sell a Linux version but not support it is pretty simple- just offer refunds for anyone who has problems. 0 work and problem solved.
Also, good Unity developers know there are platform specific issues (and how to solve them). I'm not saying platform specific bugs don't exist. They're just rare. When they happen, post on the Unity forums or Unity Answers site. Unity employees will read it, and get back to you. The support is great. This is why you use Unity. Let Unity do this work for you!
(At least, that's how it sounds to me.)