In my opinion, “unkown” is not a good way to indicate multiple species, since often enough the species are identifiable. If you just go to a high taxon, somebody else will come along and ID the first picture, which then yet somebody else will agree to.
Using Life, as @kiwifergus suggests doesn’t help either, as it will keep the multi-species observation in the needs-ID pool.
Better would be to have a way to indicate that this observation can’t be improved. So I’d like to invite you to vote on this related feature request.