Fortunately HMRC's opinion of what falls under IR35 is no more legally enforceable than yours or mine. The real problem is that they can tie you up in knots and

Fortunately HMRC's opinion of what falls under IR35 is no more legally enforceable than yours or mine. The real problem is that they can tie you up in knots and uncertainty for years of investigation even if you've done nothing wrong and then not even compensate for the wasted time and distress afterwards. Any one-sided power balance can clearly be abused if you upset the wrong officials with too much power and too little accountability for how they use it. That is a much wider problem with government than just IR35 investigations or even just HMRC though. It's not that "you can't win" because of course many people have beaten HMRC in IR35 cases once their case was finally properly decided. The big three criteria that are famously used to suggest a contract is outside IR35 (substitution, SDC and mutuality of obligation) don't come from the legislation but from early cases that HMRC lost. But it might be a rather Pyrrhic victory if you've still spent years of your life stressing about the case before it gets to that point.