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Well there's one. If people tend to compile natively much more often than cross-compile, then it would make sense to spend time optimizing what benefits users.

Yes but you probably would make those optimizations in C code and not assembly. The amd64 compiler is basicially the same C code whether or not it's been bootstrapped on armv8 or amd64.

Well to get a little nuanced, it depends on if the backend for B is doing roughly the same stuff as for A (e.g. same optimizations?). I have no idea if that's generally true or not.

There are some small nits, where representation of constants etc can be different and require more work for a cross-compiler.

Endianess differences would require more work, across all pointer math output, etc., though maybe still not significant.