The biggest trade barriers between Canadian provinces are labor licensing laws. The biggest trade barriers between American states are labor licensing laws.
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.
Not comparable at all. The US is one market unless you have a compelling reason to protect your state market. Canada is 10 separate markets with provincial agreements between them - all protecting themselves. Easiest example is Quebec.
I.e. Opt in vs Opt out but more complicated.
The problem is the differences are asymmetric. Canadian labour mobility is a lot more impactful when the pool of skilled labour is just overall smaller.
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.