In the past, the supposed fear has been about "safety", if Canadian drugs have passed as many trials and are as safe as those sold in the US. Lots of fake drugs have been offered for sale from Canada, as well.
But I believe this is a straw man and is misrepresenting the actual issues.
The regulatory burdens are different in the US and Canada. Right now Canadian companies can import drugs into the US, if they meet all the strenuous FDA requirements. In reality, that is so difficult and expensive it is often not a practical option for these Canadian companies.
It seems what Biden's proposal would really be trying to do is give foreign companies an unfair advantage over American companies. His proposal is almost like an admission that much more stringent American regulations are keeping drug prices high. Because American companies are certainly not going to be given the option of getting their products certified in Canada instead of having to go through FDA rules.
It seems to me, if the US really wanted to allow drug prices to be reduced, they should reduce the regulatory requirements that the FDA has on pharmaceuticals, or modify those regulatory requirements to make it easier for companies based in foreign countries to be able to comply, making more standardized rules across international borders. For example, maybe the US, Canada, and the EU could all get together and agree on a similar set of core regulations. Then if the US had any additional more stringent requirements, have those be something that would be easy to springboard off from those regulations which that foreign company already was required to meet under its country's rules, instead of having completely different requirements.