1. Looks bad but We won't know what this is until the bill is out.
2. If this is a negotiating tactic, it's a bad one.
3. Those implying this will sink future UK FTA's with 3rd countries are (in my view) overstating a bit.
In my view, 3rd countries like Canada and Japan will consider the WA distinct enough from regular FTA practice not to begin questioning whether the UK will live up to tariff bindings or other FTA commitments over this.
Yeah, that might be a problem but:
a) The test for Pelosi and Irish-American Congressfolk is going to be if the border descends into chaos and violence again. Not clear this does that.
b) US deal not even remotely close anyway.
Predicting how a dozen different governments will react to an as-yet unknown change to the WA is only ever going to be wild supposition. I may be wildly wrong.
What I'm pretty sure about is the EU won't like it, and they're who matter atm.
Yes, the US may (depending on how badly this breaks the GFA, we don't actually know what the bill is) refuse an FTA with the UK.
However, that'll be over a specific concern, not the fundemntal question of UK trustworthiness.