"Less than absolutely total standard compliance" != "a significant challenge compared to absolutely total standard compliance", especially if the differences are clearly documented.

The challenge is not being able to write idiomatic ANSI C, rather trying to tame the compiler to produce code comparable to hand tuned Assembly to fit into those processors, the majority of time using compiler specific extensions.

You're sounding like a propagandist - like you want to just argue your talking points, rather than actually have a conversation where you listen to what the other side is actually saying. It makes you a real pain to talk to.

Just in case you're actually trying to engage in good faith, though, I'll try this one more time. If I have a compiler that is less-than-100% standards compliant, I may not be able to use a few features of the standard. That means there may be a few ANSI C idioms that I can't use. Of those, the number that I would choose to use on that size of processor is very, very few. So in practice, there is no "challenge".

Why would I use very few of these features? Because on a processor that size, you're not writing a huge app. You don't use all the functions in the standard library, you don't use all the keywords, you usually don't push the language very far at all. At worst, you might have to develop one or two idioms of your own. It's... mildly annoying, rather than the big deal you're trying to make it.

It is not a challenge, it is by far the most common way to program them.

Yeah not being able to write idiomatic ANSI C, rather trying to convince the compiler to produce code comparable to hand tuned Assembly to fit into those processors, the majority of time using compiler specific extensions.