There's this myth that automated spam detection is hard because spammers are all very clever masters of disguise.

No. Spammers are stupid as a shoe. They have dog shit for brains.

Automated spam detection is hard because the line between spam and "legitimate" marketing activity is a fiction.

@danslimmon As someone who has been dealing with spam in wiki environment I think this is a bad take. Automated spam detection is hard because we don't want to accidentally target legitimate users. And legitimate user activity sometimes may look like spam to a dumb machine code.

I can only guess what you mean by a "legitimate marketing activity", however implication that it's easy to make automated spam detection system that would target all corporate advertising is silly.

Context is always important and no automatic spam filter can judge it better than a creature can.

I do agree that spammers are insanely stupid, that I did notice after years of working in the field.