"Fully encrypted, with the key derived client-side from your username and password" is easier said than done. It means that the key is as weak as your password

"Fully encrypted, with the key derived client-side from your username and password" is easier said than done. It means that the key is as weak as your password and that you need to download a special client or plug-in on every device you want to read your files from. That key will almost certainly be cached and persisted on a device. Otherwise, you'd need to enter a password every time you need to decrypt a blob of data. That means you need some way to revoke a key when you lose a device. You'll also need some key recovery mechanism when users inevitably forget their password. Sharing files effectively becomes a key distribution problem. Another consideration is that you can't just upload diffs of files when they change or easily perform data deduplication.