[sz]

It's [edit: not exactly] SafeSearch:

Q: If an offensive or lewd word is a fraction of my query, will Google push these results in front of me as I type?

A: As always, we provide options to filter the content you see in search. You can choose to set SafeSearch to filter out explicit content, and parents can lock SafeSearch to the strict setting. In addition, autocomplete excludes certain terms related to pornography, violence and hate speech.

[acqq]

AFAIK not SafeSearch but "in addition," the second part of your qute.

[ugh]

You basically have to press enter if you want to search for a term with possible pornographic, violent or hateful content (that’s what they just said in their Q&A). That seems like a sensible solution to me. Safe Search doesn’t figure into it.

[sz]

They probably do something like: if a search term returns a high density of SafeSearch-flagged top results, then don't autocomplete.

[ithkuil]

and if it doesn't autocomplete in the plain old search suggestions feature, then it will not appear in the instant results, because these are precomputed results for queries that are generated by the suggestion engine.