So negative cost electricity tends to be a side effect of large thermal power plants that can't ramp down quickly due to a drop in demand. Renewables can typically be turned off relatively easily. Maybe there will be situations that might still happen with an over build of renewables but I haven't heard of any. But either way if prices of electricity become very cheap then using that electricity directly with a heat pump or resistive heater also becomes cheaper.

I am not saying making hydrogen can't be economical utilizing cheap off peak rates. I just don't see it as economical for home heating or typical consumer transportation since there are alternatives that utilize electricity directly.