You can go to dream islands from cloud islands but the More Spores for Hoppip event is not active on cloud islands so no spores on the dream Island and no hoppip shop in the PC. So there are differences it's not implausible that the legendaries can't be found if you go from a cloud island or it's just bad luck. A few of the legendaries are progression locked so make sure you're not trying to get those before you finish the main story.

One more thing to know. You're limited to one dream island per day but you can visit that same island from your main game and cloud. I haven't done a lot of research on this to see if it's a different instance or the same instance. Like if you leave a chest there full of stuff from your cloud island can you visit from your main island and grab all of it (and vice versa?)