> High paid professionals with multiple clients per week (like most fractionals are) are not the target.
Yes, that's also what used to be the common knowledge in Germany. However (according to my tax advisor), the tax office here has gotten increasingly agressive and are now also targeting high paying jobs like physicians and software engineers more and more.
> We're still paying taxes, the only difference is that the client isn't paying for it
Yes, that's also how you in Germany would optimally protect yourself against fallout from false self-employment (= backpay of evaded social security payment), but even in that case your customers can be fined for engaging in false self-employment, which will likely kill that customer relationship. One practice against that I've seen is to have a clause in the contract where you as the freelancer guarantee that you also spend X time with other clients, which should give the customer a shield of plausible deniability, but I haven't heard any experience reports whether that holds up in an audit. The most bullet-proof method seems to be to have at least one employee that you pay minimum social security for (e.g. working student or family member), but that's not trivially cheap.