Well, the fact that Google officially states that underscores are now considered as separator does not means it's the best one.
I personally keep hyphens because they once where the only one to be considered as separators, and thus are more likely to work for most bots and Search engines.
Then, it's as well a matter of language, an hyphen is a genuine word separator, where underscore are never used when writing.
So at first, bots behaviour was pretty logical, treating things like "super-seo" as two words and "dll_32" as a single symbol.
So IMHO for the most, it's the same, but it still could be better to keep hyphens for some SEs.
I don't think that if an SE would penalize too much hyphens in some URL, it would no be the case with underscores.