Substitution was always a good one.
Initially this was posited as using kit provided by the "employer", course that was a nonsense as not many serious organisations let you plug personal kit into their network.

Another one was most contractors didn't legally have holidays or sick pay, etc, unless they provided it themselves. Seeing as employers and the state have to meet statuary minimums in the UK, they had to mess about again.

While I'll be the first to admit pre IR35, a lot of contractors were taking the urine, they went way too far with this and threw the baby out with the bath water. I also find it typical that they went after the little guys.