This may be a regional thing. I’m in Austin, TX where it’s often hot even in February. I needed to get some of my rental properties to meet Section 8 (subsidized housing) standards, and one thing I had to do is add vent fans to each bathroom. The cheapest and easiest to install were those that vented directly into the WALL! Even in an uninsulated wall you’ve go a pretty small space to disburse the air. I even called the Section 8 people and told them I didn’t think that was a very good idea, but they assured me that it passed the local codes and was OK.
I ended up installing them in the ceiling where they vent directly into the attic. Reading this thread has got me worried about even that. Of course this is Texas and I can’t remember ever being cold in the attic.