> it's easy to end up doing something that is implementation-dependent if you're not careful.

I wouldn't say that. I've done a fair amount of porting code between CL implementations, and it's usually pretty straightforward. Implementation dependencies tend to be found only in code doing OS-y things like threads, external processes, network sockets, etc. This code is usually not difficult to find, and I would think that the authors would have expected it to be implementation-dependent. Computational code usually ports with zero effort, even when the machine word size changes.