[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.