When I received Hoya soidaoensis as a cutting from AH Hoyas three years ago, it was called Hoya sp. Khao Soi Dao named after Khao Soi Dao, Chanthaburi Province, Southeastern (SE) Thailand where it was discovered. The plant flowers, while similar, are different from Hoya caudata and H. flagellata in one important way – they stay open all day and all night. Hoya caudata and Hoya flagellata flowers close up almost entirely during the most of the daylight hours. The same is also true of another close relative – Hoya phuwuaensis; the flowers close during the day. So from a standpoint of flowers alone, Hoya soidaoensis is the superior plant!
Hoya soidaoensis is not a plant for beginning Hoya enthusiasts. I struggled mightily with mine having to start it over at least twice. All of my travails with this plant can be found in my blog, but I will outline them here. This is a plant that never wants to dry out, or be too hot while in bud or the buds and peduncles will abort. That being said it also is extremely easy to rot the roots out while the plant is young. To help avoid a root rot tragedy, I recommend growing it in a net pot inside a sleeve-pot so that it can be easily pulled out for observation and watered just when the mix is almost dry. For questions on this procedure, see my video on growing Hoyas in net-pots. As long as it is not to warm (above 90 deg. F), and it is kept moist, the plant is much more forgiving when mature.
Some people get lucky with Hoya soidaoensis and flower it early and at a small size. I was not so fortunate in that it took 3 long years and much trial and error. It was however worth the effort. The plant is not for beginners, but with its beautiful flowers, I believe it to be a worthy addition to the collection.