A good accountant will make suggestions about things like control over work and deliverables but they can't tell you for sure if a contract is Inside or Outside.
They don't know that because HMRC don't publish that information and their online tool only makes suggestions that you maybe inside or you maybe outside.
You might think "just operate like a business not an individual" but that's not as easy as you might think.
IR35 applies per contract not per worker. So having multiple concurrent customers doesn't protect you.
If, say, you set up deliverables for a piece of software, deliver it to schedule and provide support during a testing period.
Is that Inside or Outside IR35?
The work has deliverables so that sounds like you're operating Outside. Hang on though, it also has a time-base component where you're providing support. I'm afraid time-base is probably Inside.
Your accountant does not know for sure and they won't provide you with any real form of indemnity for their advice because of that.
Netherland also still doesn't have clear rules about that. A couple of years ago they tried several things, and people and companies rebelled against some of those, and now they're mostly ignoring the whole issue again, I think.
At least in my field, where contractors are paid far better than employees. There are also obvious, blatant cases where companies do use this as a loophole to reduce costs. And sometimes they do that with permission from the tax service, even! PostNL, the Dutch postal service fired all their mail deliverers and hired them back as contractors, but they still had to follow the schedules set by PostNL, wear their uniform, etc. Yet the tax service approved that.
I don't get it. I think that's a blatant case of the abuse they're supposed to fight. But at least they're leaving me alone for now. I think the unspoken rule is that if you're paid well, it's probably fine, and they're focusing on cases where people are underpaid and unable to save up for retirement. Because those are the cases where this is going to cause problems.
A good accountant/tax advisor can help here. They do know how HMRC categorise disguised employees and can help you not get burned.