I tend to feel that way about much of the series. It does some really great things, and I love the show for them, but it also does really irritating things like the belches that really wreak the show for me. Then I get frustrated at the ruined potential.

I don't know.. Sadie kinda demonstrated that Gem monsters at least can be killed with a pointy stick. There were groups of humans 5000BC capable of organization, urban areas had specialized soldiers, and copper weapons were accessible in some regions. Humans could probably pose a threat at that point.