I work as FTE and I went through the employee tax policy document of my employer and in that they have clearly mentioned that taxes go by the place of employment. There is no mention of residence anywhere. From that what I understood is as long as one commutes to their workplace at some frequency, there is not going to be a HR or tax compliance issue.
Regarding employer tax obligations.. in addition to above point, I learnt that a US employer has no obligation to deduct payroll taxes outside US. They are obligated only to IRS and not to CRA. It's employee responsibility to take care of taxes for the place where they live.
Also, CRA does recognize people who live in Canada and commute to their work to US as 'factual residents'.
https://www.canada.ca/en/revenue-agency/services/tax/international-non-residents/individuals-leaving-entering-canada-non-residents/factual-residents-temporarily-outside-canada.html
So per CRA this is absolutely okay to do as long as someone 'commutes' to their workplace. I am not sure how working fully on a remote basis for US employer without traveling to employer workplace would pan out, but if you consider the fact that a US employer doesn't have any obligation to Canadian revenue department, it should be okay to do that in that perspective too.
There are many companies like general motors, ford , Amazon etc based off of Detroit allow people to do this.. if it was an issue, they wouldn't be allowing them right.?