While it may not be the most secure, for many casual users the level of encryption provided in cloud chats is good enough for the convenience of smooth cloud sync, which generally doesn't work with E2E messengers and I frequently lose messages when I restore my phone or get a new one. At least they say the encryption keys for cloud chats are scattered across multiple jurdistictions so they wouldn't be able to hand over anything (other than public chats) unless someone got a court order in bunch of different countries at the same time. I would definitely trust them more than WhatsApp even though the latter uses the Signal protocol, but I don't trust Meta to not collect metadata or possibly have other backdoors as well.
Smooth is not the word I would use for them. Though it may very well be only a “frontend” problem and the underlying tech/theory is probably good. But the current implementations are not yet there unfortunately.
