different methods for hardwood cuttings - why , which climate, which plants?

I had mixed results with hardwood cuttings, but I simply stuck them in pots and left them in the open.
That is actually what some sources suggest (mostly in the ground). Others say you need to put them on heated benches which I don't have and others say that you put them into moist medium during winter and once they callus you pot them up. Others put them in a plastic bag.
So why would you use which method for what? We get nights with about -5C in winter. But the days are usually warmer, say 10C.

I'm far from an expert but I watch Edible Acres on Youtube.  He does a lot of cuttings and propagation in his permaculture business.  I think he mainly does hardwood cuttings in compost amended soil in the ground.  He's in New York so much colder than you and it seems to work for him.  I did my first hardwood cuttings last fall (stuck them in a garden bed) and some are starting to break buds.  

It really depends on the plant you're trying to propagate. Some are fine just sticking them in the ground. Some will mostly fail if you do this, while working out quite well with a heated root zone. Some are better off propagating softwood cuttings. Since I'm not an expert in all of this, I bought a book to help me along - The Complete Book of Plant Propagation. When I want to propagate something, I look it up and see what works best.