Actually Linux distributions are binary compatible with eachother and the LSB mandates that they support the old ABIs(Which means they are always backwards compatible), drivers are a different matter since the kernel interfaces change frequently, but that goes for any OS, (Microsoft have changed their kernel interface with almost every single kernel release they've made and it breaks driver compatibility almost every time.
Karsten_, on 09 Feb 2013 - 19:13, said:
Linux doesn't really maintain backwards binary compatibility in the way Windows does. (Which is why NVIDIA and AMD's drivers tend to be problematic).
If you want to use proprietary drivers in Linux and avoid problems the only thing you have to do is use the kernel that ships with the OS. (And use an OS that doesn't push out new kernel versions as part of their normal update routine or atleast one that installs new versions of any proprietary driver you're using at the same time)
The fact that Unity3D doesn't work well with RHEL6 has nothing to do with backwards compatibility, glibc is backwards compatible these days(it wasn't back in the 90s, but this isn't the 90s) but old versions of glibc does not magically support software that requires newer glibc versions. (Just like you can't run a game that requires D3D11 on Windows XP).
If you want to run modern software on Linux, do not use RHEL, it is ancient before it gets released(RHEL7 should be able to run games made with the current version of Unity3D). Its great if you need stability, but if Microsoft did like Redhat the latest Windows release would be Windows 2000, SP13 and the only feature updates we'd get would be for things like Hyper-V or MSSQL.
Personally i wouldn't use Unity3D to target Linux today though, if i sell a linux game i have to support it, and offering a unsupported linux client to those who buy the game for Windows or Mac is pretty pointless. Seeing how much problems Unity3D has had to get Android support working reasonably well i'd prefer to wait until others have run over and exposed most of the pitfalls, once that is sorted i might consider supporting Ubuntu and possibly Mint.