Your cousin would have been a PR in Canada as well as GC in US hence the issue. You cant make both country a primary residence to show the respective country that you are trying to settle permanently.
Definitely not all officers on both borders are comfortable with. My friend who mentioned India address as permanent address by mistake was warned that if he wants to give up gc. So surely the gc candidate's intention should be to make the US a primary residence, not some other country.
So above situation is not comparable with the situation i mentioned. Here the candidate is on H1 and will be PR on canadian side. Both cases are totally different. We are commuting for years without a single event on retaining immigration status. So you can broadly tell that commuting is bad idea.
No red flags in commuting. One of my directors crosses border on commuter gc for years. No issues. We all commute on H1 for years. No issues.
Intention clause for canadian citizenship is gone few years ago itself. I agree that one should stay inside till citizenship but its not practical/economical & unnecessary for most to give up job at the border and move.