No Cloud based method is 100% safe.

Follow these Cloud Sync simple rules:

Every computer you are syncing should save zip backups to the same cloud folder, if you are not syncing live Projects with Dropbox.
It is best to have ALL/ANY cloud backup service set to store files to be available offline. (IE available at all times on your computer) This will avoid many sync issues with cloud services for your project. (Discussed below)
Enable saving backups with zip compression. A zip backup is essentially one file which contains everything inside the Project folder.
You should ALWAYS keep your zipped backups in a different folder from where your live project folders are stored. Ideally, the zip backups are saved on a cloud service as an additional backup option. (see below for a detailed discussion on several common cloud services) In this way, live Projects are stored locally on each machine and updated from a shared cloud based zip backup folder.
Strongly consider using dates in the backup name.
Ideally, save as many backups as your cloud storage allows, this helps if a sync issue occurs. When syncing between multiple computers, the computer with the lowest number of backups sets the limit for the other machines. Make sure all the sharing computers are sharing the same number of backups.
You should also consider storing recent zipped backups on a USB key or external hard drive as an additional safety measure when actively writing to a project. A little extra time to use the Menu command File > Backup to function to offload a copy of your project is well worth it.
For each computer, your live Project folders should be in an area that is NOT synced to the cloud and different from the backup location.
Allow the project to sync to the cloud before shutting down your computer. The dropbox app has a sync icon on it to show when the process is complete. Closing the computer early before the sync process has completed can result in errors.

There are two ways to backup Scrivener Projects to the cloud:

Method 1.

As a live folder project containing all the files. Per the L & L site ONLY Dropbox syncs live folders reliably.

Method 2.

Using a Compressed copy (zip copy) of your project for backups.
Zip backups can be reliably saved on most cloud services.

Copying your updated project’s zip backup to the local machine:

Extract the latest zip backup for the project and choose a location to place your Project Folder outside the sync folder.
I put them on my desktop, then delete the "old" Scrivener Project on the computer, then move the "new" Scrivener Project from the desktop into my Project folder where my local live projects are stored.
Open the updated project the normal way using the Menu command File > Open. Select the project folder and double click on the *.scrivx file to open the project.
Following step 2 ensures only one working copy of a Scrivener Project is on your computer at one time. Though this method requires more steps, it greatly reduces the risk of syncing issues when using Cloud services including iCloud, OneDrive, and GoogleDrive among others.
You want any Cloud service you are using to also save your files offline (locally on your computer) for additional safety and in case you are using your computer in a location without wifi, or stable wifi access. This ensures you have a usable backup on the computer/device at all times.

Setting different cloud services to save its files offline:

For Google Drive

Right-click on the letter of the Google Drive (usually G), then see the My Drive Icon. In the right-click options look at the option Offline access and make sure Available offline is checked.

This Google support Page shows how to use Google Drive Files offline.

For Microsoft OneDrive

To make a OneDrive file or folder always available on your computer offline, right-click on the file or folder and select "Always keep on this device".

The file or folder will now display a dark green tick. (see above) These always available files download to your device and take up space, but they’re always there for you even when you’re offline.

This Microsoft Support Page explains how to keep One Drive files always on your computer.

For iCloud

Install the iCloud app for the PC from the Microsoft store (free download)

Sign in with your iCloud password and ID

You will be asked to verify the request to install and enter a code. You will receive an email as well about this.

The screen will give you options as to what you want to add to the computer.

After installing iCloud you will have two ways to ensure your iCloud files (including Scrivener backups are stored locally on the computer.

Method 1:

Open the app. If you open the app and will see the iCloud Drive option with a Caret to the far right. Click this and make sure the iCloud Drive option that files will sync to their own folder in File Explorer is clicked on. You will also have the option to change the default storage folder. (see the image below)

Method 2:

Go to the iCloud drive icon in your File Explorer and right click.

Right click on folder and at top of the list chose Always keep on device.

Any changes you make to the file will automatically upload to iCloud Drive when you're back online.

Here is a link to the apple guide for this-

For Dropbox-

There are two methods to store the files offline on your computer:

Method One-

You can right click on the Dropbox folder on your computer after you have installed the desktop app. Now you will see Dropbox options to Make files offline or to Manage Hard Drive Space. If you click this, you can view your Dropbox items as individual files or as folders (Use the folder view option).

In the situation below, Dropbox is already storing your files offline. This is shown by the green check next to the option Dropbox on hard drive. If it was online, you would have the option to make any folder you check offline as well.

Method 2-

Right-click on the app icon in your computer taskbar.

Click on the your avatar/name in the right upper corner of the newly opened screen.

Click Preferences and then click the Sync tab in the newly opened screen. (See images below)

The Sync screen gives you multiple options. If you have the room, choose Available offline and all the files will be available locally and in the cloud.

Note computer backup services like Carbonite, IDrive, etc can also back up scrivener files to the cloud. Make sure where you store your zipped backups are included in the folders/files being backed up by these type of services. All Scrivener files should be backed up multiple ways including externally to a hard drive or USB key as added insurance for these precious files.

Overview on Cloud Backups