Mine is in a shed but at night I shut her in her hutch in the shed - just to be sure.
We kept chickens for years and the only time we lost an entire hen hut full to a fox was when my dad forgot one night to put the catch down on the little cat flap style door. The fox must have got in via the attached run, so runs can be weak points too. He lost 30 hens that night and I think only one was even eaten. He never made that mistake again! So yes, the doors are the weak point but don;t underestimate how small a hole a fox can fit through either, as this wasn't the main shed door, it was the little cat flat type thing at the back. And it wasn;t a ramshackle thing - it was a substantital, purpose built hen house not as flimsy as a modern shed.
That's why mine is in her hutch IN the shed, as soon as it's dark! And so far she still has a separate freestanding run, I can keep an eye on in the daytime - as that means I don't have the weak spot with a run door, or a cat flap style door. My shed is also on concrete, and the floor is covered in lino that is nailed down under battens the whole way round.