At first glance, this seems a little underpowered, but as I began to ask myself why that was, I really couldn't answer that. There is some very subtle stuff going on here. The only thing that jumps out at me is Nonresistance. Since it specifically says Strength attack bonus, I would guess that it does not work against creatures/beings that us Dex as their attack bonus? (I state this, fully realizing that 90% of most enemies you face in DnD use Strength as their attack bonus).

Other than that, I say this: im not exactly an expert in balancing Homebrew but I have to say this 'feels' really good. Can't wait to see this evolve!