Juncos (aka snow birds) prefer to feed on the ground, but they will also visit platform and hopper feeders. They eat just about every available type of feeder food including black oil and hulled sunflowers, safflower, millet, Nyjer, cracked corn, millet and peanut halves.
If you look under your feeders after a snowfall you’ll often see juncos jumping around snarfing up any dropped seeds from sloppy diners above.
I toss a mix of safflower and millet on the ground near some of my arborvitaes and that always attracts a flock of juncos (plus House Finches and House Sparrows).