I have 49 collections
. I have had most of those for many years and others keep getting added. I have the O3, O1, Voyage that can use those cloud collections. My K3 and older basic of course cannot. At times I use the kindle app on my android. I have made collections and added books to them on all those devices including the webpage. All my collections show up on all my kindles. I can switch them out as I need to, if one of the other is closer. I keep the same collections "starred" on them so its pretty much the same. It takes under a minute at most for them to update when I add or delete books. Rare are the times where it takes a bit longer, but it does happen.
I have a few collections that have up to 700 books in it. I try not to go over that though so i always start a new "read" collection when I get close to that. That does take a wee longer, but not as it used to do. The older kindles could take like 3 minutes to add or remove a book from such long collections. Now its just second. And it all syncs across my devices perfect.
So it works fine for some of us. I wonder why it doesn't for others.