I've found too often that by the time I figure out it doesn't fit and needs to be exchanged, other sizes are sold out. It's doubly annoying because I either have to eat the cost of an item which doesn't fit or eat the cost of sending it back and losing the discount they give you for keeping everything.
I like stitchfix quite a bit, but I'm starting to become a bit disillusioned after seeing the small bag of clothes dropped at the Salvation Army which didn't fit or I didn't like and which it was cheaper to keep than return. At least I have a mortgage and this itemize deductions, so I can write those clothes off my taxes!