He summarizes the plan as follows:
The central concept put forward by Mr Ryan, which appears to have the support of Mr Trump, is to turn corporate income tax from a tax on the return to capital into a tax only on extraordinary profits. This would be done by taxing corporate cash flows. In addition to the major reduction of the overall rate, the system would change in three fundamental ways. First, all investment outlays can be written off in the year they occur rather than over time. Second, interest payments to bondholders, banks and other creditors will no longer be deductible. Third, companies will be able to exclude receipts from exports in calculating their taxable income and will not be permitted to deduct payments to foreign suppliers or affiliates from income.
I found this to be the paragraph I had not seen elsewhere:
Second, the tax change will capriciously redistribute income, increase uncertainty and place punitive burdens on some sectors. Think of a retailer who imports goods from abroad for 60 cents, incurs 30 cents in labour and interest costs, and then earns a 5 cent margin. With a 20 per cent tax, and no ability to deduct import or interest costs, the taxes will substantially exceed 100 per cent of profits even if there is some offset from a stronger dollar. Businesses that invest heavily, hire extensively and export a large part of their product will have negative taxable income on a chronic basis. It is hard to imagine that the political process will allow annual multibillion-dollar refunds, so they too may be victimised. Then there are the still unresolved questions of what the rules will be on interest deductibility for banks and of the treatment of businesses organised as partnerships that do not pay corporate taxes.
Here is the FT link, probably gated for most of you, WaPo link here. Summers also argues the plan will worsen inequality, strengthen the dollar (possibly leading to EM crises), lead to a trade war, and erode the long-term tax base.