what if I backed up to the Protonmail cloud?

I realized on my own that backing up to some cloud such as Google's or Amazon's was not the best idea, even before I became anti-Google and anti_Amazon; but I note that Protonmail, which I use daily, offers a cloud drive with its paid subscription. (Or will offer--I think it was in beta last time I saw the subscription offer.) I'm happy backing up physically anyway, but if I wanted to use a cloud, would Protonmail's cloud be a safe choice? Or only less unsafe?

I've thought about subscribing anyway, so I can make as many folders as I want. So, I suppose the question might be phrased as: if I subscribed, should I use the cloud?