For what it's worth, the various forms of venting can all work, or not.
I was a roofing project manager (PM) for a Design/Build contractor (thrown in to the deep end with no training - yay!). This was added on top of my Home Performance PM job duties. This was in Bend Oregon right after we'd had a winter that led to a lot of houses having destructive ice dam issues. There was 3-1/2 feet of layered snow and frozen rain on my roof that had to be shoveled off - that happened in 5 storms over the course of 1-1/2 weeks - oh joy! The houses that I saw the least damage in were the older houses with gable vents. The ridge vents were sealed up with the above-mentioned snow/ice layer cake. There's a fair amount of wind here regularly, which is my best guess as to why the gable vented houses worked better.
The GAF roofing materials representative argued the company line that we had to close the gable vents - because they were neither high or low vents. My theory is that it doesn't matter (gable vents don't care what you call 'em) if air moves through the attic regularly - and I argued with the GAF rep endlessly. I understand the idea, but our local experience said that the gables were just fine. If you have to have the roof shoveled in the winters when we have those kinds of storm, it's not exactly beneficial, especially if the occupants are elderly, or if the rooves are steep or 2-3 stories high or otherwise difficult to access. Bottom line, you need airflow under the sheathing, by whatever means works in your situation.
Building science is a discipline that needs to be applied with thought to the individual scenario.