What about pronunciation? Many of the assertions I've heard about adults in a foreign language is about our ability to recognize, differentiate, and reproduce the different phonemes, many which do not exist in our language.

These phonemes are even more difficult to recognize when we're not conversing face-to-face and in-person! So if you're listening to "comprehensible input" if it's on audio, or video voice-over, it is much inferior to seeing/feeling/hearing a native speaker make sound-shapes with their mouth!

I made many efforts to imitate my Spanish teachers in my youth, in terms of pronounciation, mouth shapes, accent and emphasis, etc. I credit the in-person instruction with achieving a nearly fluent comprehension and ability to make myself understood.

So the argument goes: if an adult is set in their ways and knows a particular set of phonemes, (or even tones, etc.) is it more difficult than a blank-slate child who has no prejudice about hearing and learning new sounds?