The second biggest trade barriers between Canada provinces are liquor regulations. The second biggest trade barriers between American states are likely car emission laws.
Fairly comparable, IMO.
Doctors can move pretty much freely between 44 American states. Doctors can't do that in Canada, and here we need more specific kinds of doctors in rural populations and there is absolutely no reason to not have a harmonized licensing regime.
We also have non-uniform regulation on stuff like the manufacture of construction goods (for example door sizing) which can cause a big drag in trade. This might be harder to fix but it exists and is not comparable to the US market.
I.e. Opt in vs Opt out but more complicated.
Of course it's more complicated -- the US's inter-state commerce clause has been broadly interpreted by the courts.