There are impact assessments in the Red Book. There will be relevant impact assessments before Second Reading, as there always are.
The key point on tax credits is what they got for all of that: unsustainable spending that went up jerkily, but by huge amounts; and a subsidy for employers, which enabled the payment of lower wages and completely distorted systems, and presented a bizarre set of incentives for moving in and out of work. It is now well documented that for many people it made sense to work only 16 hours —no more, no less—and we saw spikes in the employment data at 16 hours. There were huge spikes of people clustered around 16 hours, because it did not pay to work anything else.