I can't really imagine how the enforceability of the border adjustment tax is going to take place, again, because of the extraordinary amount of paperwork that is required and because it is done at the business level.
I don't see that it could be compatible with the WTO commitments. The WTO has long evaluated various types of the value-added tax system. Canada's GST is considered to be WTO compliant because it doesn't violate the national treatment rules because it does not function as an export subsidy, but the border adjustment tax would seem to be in violation of both of those things.
If that is the case, the U.S. has said, well, maybe we'll just ignore those rules, maybe we're just going to barge ahead anyway. Other global trading partners have come forward and said in response that they will retaliate with their own countervailing duties, and the estimate from the Peterson Institute for International Economics—and you can go on their website and find this document—is something in excess of $100 billion in retaliatory tariffs that the U.S. could be vulnerable to, including some $40 billion from Canada.
Again, by almost all measures, including the President himself saying he doesn't love the tax, it does not seem to be a winning policy, but at the same time, it is a fundraiser for a government that desperately needs funds, so they're loath to get rid of it.