I have a 4 in 12 pitch roof and have leaf guard (or something similar, can't remember brand). It's been up a year. So, far it handles the pine needles and oak leaves (and twigs and berries) just fine. Only in the heaviest rain, approaching an inch per hour, do the valleys have so much water that it overshoots the gutters.
But, there is a concern you ought to look out for. I live just below the snow line here in the California Sierras. Which means we get snow once in a while and regularly get hail from the T-storms that go through. Last year we had a hail storm that produced about 1/2 inch of pea sized hail. It rolled off the roof and accumulated on the top of the gutter guard/top. Some hail also flowed over the edge and plugged up the small gap between the cap and the gutter. The rain that came with the hail wasn't warm enough to melt the hail for several hours.
So, with the water gap frozen shut, all the rain that also came with the storm overshot all the gutters along the whole length of the gutter. This then splashed in the yard and kicked up mud on the lower house walls-an event we never had before. Previously, when the drains plugged we had water over flowing in one localized area--not the whole length. Only a small amount of the water flowed around the curve and into the gutter when the hail was stuck to the top and gap of the gutter. I'd suspect the same would happen with snow.
I'm still happy with what I've got, we only get that much hail once in a while. But for you in the snow belt, I'd ask a few questions of the salesman.