I shoot weddings and edit with LR Classic. Each wedding gets it's own LR catalog to keep things simple and together. I use LR CC on my iPad pro and desktop for storing and editing personal photos. Sometimes I would like to edit my wedding work out and about and my iPad running LR CC would seem like the perfect solution. However, if I sync a collection from within a LR Classic wedding catalog with LR CC, it uploads the collection to allow me to edit in LR CC but that same LR Classic catalog then downloads my entire LR CC library and collections, introducing personal pictures to my streamlined and unique wedding catalog. I appreciate they are two completely different products, however a simple option to only sync a specific collection from within LR Classic would make editing on the go a breeze. There are enough dialogue boxes that pop up after you have synced that prevent you from deleting images mistakenly from the "all synced" collection, a simple check box before initiating the sync asking if you would like to sync/download the entire CC catalog with this collection or not would work perfectly. Or even an option in the sync preferences:
sync all CC contents with this catalog - Y/N
sync only selected collections with CC - Y/N
In this scenario, every new catlog I make could have a "to edit mobile (insert couple name)" collection that I drag and drop images into when ever I need to edit on the go, without downloading my entire CC library to every catalog.