What you always have to consider with farfetch is that it isn't just ONE shop, but a bundle of many actual boutiques from all over the world. This of course means different conditions, shipping costs and return policies. I agree they should have a standard shipping rate and return policy, but at the same time I understand this might be difficult to achieve.
I have ordered from them before and never had any problems. But I always check where the item I want is located, and so far only ordered within Europe. That way, shipping was low and returns wouldn't have been that much of a hassle.