It's a problem anywhere, just that the dry bulb temperature needed to reach a given wet bulb temperature goes up as humidity goes down.
Earth would have to experience > +35 to +50C for the poles to be uninhabitable due to heat.
Not reliably, not continually, and much less often when you dump enough energy into the atmosphere to disrupt major wind patterns.
British Columbia hitting 121°F/49.6°C at 50°N latitude would sort of suggest your generalization doesn't hold true anymore.
Here's a citation demonstrating that over the last 95 million years if you need one: https://www.pnas.org/doi/10.1073/pnas.2111332119
One more just for fun: https://journals.ametsoc.org/view/journals/clim/12/4/1520-04...