Of course the biggest problem being that "inside /outside IR35" is not (and has never been) clear. If HMRC had their way, every contractor is a disguised employee and inside IR35. Their CEST tool has been criticised for being biased towards finding people inside IR35, and not taking all of the relevant factors into account (eg. MOO). So contractors relied on expert advice to make IR35 determinations. HMRC made their own and took people to court. A lot of contractors (especially lower paid, who could not necessarily pay for expensive advice) were lured in by so-called financial advisers offering remuneration via loans, to avoid the entire IR35 debacle. HMRC remained largely silent for 20+ years about these loan arrangements, trying (mostly unsuccessfully) to take some schemes to court more recently, but providing letters to contractors who asked them about the legitimacy of these schemes saying they are fine (yes, this is true - google will allow you to look at some of them, they have been posted on twitter) - thus tacitly approving such arrangements (and let's not even get started on DOTAS). Then in 2017, they claim "they've always been clear" the arrangements never worked. Despite having to introduce a new retrospective law in 2017 to be able to recover the tax they say is due - if they never worked, why the need for a new law? My advice is that take whatever precautions you can, but ultimately it seems HMRC are acting in bad faith. They can and will (and have) bend the rules in their favour to get the outcome they want to achieve.