I tried to root a lot of cuttings of C35. I was usually successful and I got very well growing plants with strong root system. I put the cuttings sometimes in loamy soil, sometimes in compost earth, placed a plastic bag over them and put them in a shadowy position or half shadow. I made most cuttings during spring and summer, but some in autumn (indoors). Success rate was really high and I am so impressed by the good development of my cuttings that I would never waste time in growing C35 from seed.
According to my (limited) experience C35 is one of the best growing rootstocks for potted plants (besides Swingle5Star and Swingle Citrumelo, better than Carrizzo).
I don't know why experiences in rooting C35 are so different.