I'm not sure if it is "most". In every company I worked those were outliers (e.g. max ~10%, and in current company it is only for HR/reception workers), but yeah, it is quite common.
But not all employers allow B2B contracts and if they do, it frequently involves some kind of worse benefits than the usual employee contract. And different employers have different way of handling vacation (some don't pay for it, some do), so it is another caveat to watch for.
And B2B people are always the first ones to let go (they are hired, because of that "perk"), so you can find another job fast, but not always the one you want (e.g. further from your place).
Generally if your employer allows you to select form of employment it is quite good sign.
If the employer allows only B2B it is usually a way to cut costs for the employer and I would watch out.
As you can see I prefer normal, cosy employment contract, but yeah I have children so that probably influences my view.