LES: Perhaps it should be noted that Unix is not the same as Linux. It is very easy to find support to all things Tcl in Linux, but not so easy in the BSD Unix family. The point is, some things work fine in one, but not in the other. Especially binary things like Tclkit, Freewrap, and certainly a few extensions. These would have to compiled for Unix specifically, maybe after a few modifications.
LV: I'd say it differently, myself. There are a variety of Unix variants. There are more members of the Tcl community available to help problems in some of the variants than others. Some of the Unix variants are currently developed, while others are not. Some of the Unix variants are open source while others are not. All of these factors contribute to a challenge in some of the more obscure combinations.
If you have a problem with a particular combination of Tcl and Unix variant, drop by comp.lang.tcl and see if anyone is around who can help. Also try a web forum or usenet newsgroup for your specific variant of operating system.
makr: If you need to write things for multiple Unix variants, be sure to check the POSIX standard. It defines common behavior of tools, as well as common C-API. Then you need to look what needs to be done to get the current system to behave POSIX-compliant. That ranges from out-of-the-box to the installation of special packages with various steps in-between...