Some are more difficult than others. You have what's referred to as 'Hindu Rope,' which is a cultivar of Hoya carnosa. There are other cultivars of Hoya carnosa (all easy IMO) including 'Krimson Princess,' 'Krimson Queen,' and the plain ol' Hoya carnosa (all green leaf variety, non-curly). As to which others are easy, I think there's one called 'DS-70' and Hoya bella. Beyond H. bella and H. carnosa, I don't know enough about Hoyas to say which other species are easy.
Planto