There are indeed many cold-hardy species but the limitation is that for most, they must be dry, and the temps need to warm up during the daytime following a freeze, otherwise you are taking a big risk.

so...

is your growing area dry in the winter?

No, it gets wet here when we are not having a drought. I guess I was just jealous reading about others outside plants. I might try leaving one of my adeniums in the garage with a sunny window?
Mike

Adeniums are one of the few succulents that are discussed on this message board that are particularly sensitive to cold. Adenium obesum don't always have a definitive dormant period, especially the newer hybrds. Other Adenium will shed their leaves. Sustained nights below 50F will trigger slowdowns. Sustained temps below 40F is risking die-back of growth tips. Continued cold risks death.

Sunlight is not essential for dormant Adeniums. If the garage is too cold, keep them somewhere warmer even if light is not ideal. Of course if growth starts, then they need sun. If they are kept dry then they are more likely to stay asleep, but not necessarily. Just do the best you can, once they are back outside in the spring, the poor growth can be pruned back and all will be well.

An Adenium obesum planted next to the Newport beach coco-would rot first. I lost a large golden trunked plant to rot this winter,putting it out too early or because I left it out too late. I can pass along the knowledge that they DO NOT become hardier as they get larger.It's my worst fat plant casualty ever.

Look at some of the pseudo-caudiciforum mesembs like Aloinopsis, Delospermas, Mestoklema, and Trichodiadema. But generally plants with a caudex are not designed to be cold and wet at the same time.

Some Delosperma are cold-hardy even in Denver, and there are Opuntia that grow as far north as Canada.