Your first sentence is a perfect example of why emdashes need to exist. The comma is overloaded, since it can be used both as a list separator and for parenthetical asides. But that first sentence starts using them as a list separator, and then inserts a parenthetical aside into the list. Emdash wins this test.

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thfuran 3 hours ago | parent [-]

I think parenthesis are ideal for a parenthetical aside.

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readread an hour ago | parent | next [-]

I sometimes prefer parenthesis for very short asides—say, three words or fewer.

I find m-dashes easier to read for most other purposes. They’re nicer in longer asides because they’re a little more visually prominent, while also (for some reason) not breaking my mental flow quite so much. They’re far nicer looking when an aside comes at the end of a sentence, as you don’t need to close an m-dash. That always looks like crap with parenthesis, as if the author should have thought of a way to avoid doing that, while with the m-dash there’s no problem at all.

	▲	oneeyedpigeon 35 minutes ago \| parent [-]
		Em dashes don't nest (but that's probably a good thing).

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oneeyedpigeon 36 minutes ago | parent | prev [-]

I think it's generally accepted that parentheses are more parenthetical than emdashes.