I have two cedar trees and beneath one of them I have Vinca minor, but it vines and can become invasive. I have to trim it pretty often:
I also have helleborus beneath the one in the back garden and I'm thinking of trying a dwarf rhododendron.
And I have comfrey beneath one as well.
I don't really have problems with things growing beneath or near the cedars as long as they are plants that don't mind being a little dry at times. I do however keep the lower limbs of the cedars trimmed about 2 feet from the ground. That helps air circulate and creates a bit more light for the under plants. I have tulips and irises very near the one in back and I have daylilies within a couple of feet of the one in front, but neither of them are underneath the trees.
For me it's a question of drought, if they survive drought, they'll survive the cedar trees
