Canada is also a much smaller market that has minimal competition with unique cellular bands preventing reuse of said phones outside of a handful of other countries (without severe breakage of VoLTE, or missing low band coverage).
It's possible there are some obscure features that may not work properly but for general usage as far as phone calls, texting, data usage, I've never heard of anyone having any issues with their Canadian phone internationally, besides maybe the cost of roaming.If you have a source from recent times that says this, I'd like to see it.
As far as minimal competition goes, there is not an infinite amount of cellphone carriers in the US either that will give you a phone on credit, especially if you have bad credit. I'm sure there are existing ways you can scam phones out of carriers too if you are fine with being banned from the carrier and/or torching your credit.