No matter what media you use, be it a USB stick, a HDD, the Cloud, etc. all media will eventually fail, given enough time. Your data will not be safe unless it exists in at least three places, such as on your computer, on an onsite backup, and on an offsite backup.
All the Cloud is is a server somewhere filled with HDDs that is accessed via the internet. The main dangers to using the cloud include a possible lack of security or data loss should the service fail. Free cloud websites are notorious for getting hacked and/or disappearing with little or notice. The paid sites are far more reliable and secure.
If you are keeping data on a USB stick because you don't have enough room for it on your computer, I strongly recommend replacing the existing HDD with a larger one so you will have room for all your data. Then use something to create an onsite backup on something such as a USB stick, or, better still, an external HDD. You could use a second USB stick or external HDD for the offsite backup or use a good, paid cloud backup service, such as Carbonite.com (my personal preference), Crashplan, or Backblaze.
More on backups. A true backup is data that is replicated on another medium that is kept disconnected from your computer except when updating the computer. An onsite backup is one that is kept in the same area as the computer itself. However, since a catastrophe, such as fire, flood, theft, etc. could cause you to lose both your computer and your onsite backup, you also need an offsite backup. An offsite backup is the same as an onsite backup except it is kept somewhere away from where your computer and onsite backup are kept; the farther the better, such as in a locked drawer or locker at work, a trusted friend's or neighbor's house, or in a safe deposit box at your bank. The only downside to an offsite backup is any data you acquire or change after you last updated the offsite backup will be lost if you lose your computer and onsite backup.
Updating an offsite backup is logistically difficult but an easy way around that is to use a good paid cloud backup service, such as one of the ones I've already mentioned. Those service install a small program on your computer that will work in the background and upload any new or changed data to your cloud backup. This makes it much easier to keep your offsite backup up to date. Good cloud backup services will also have versioning. Versioning means discarded data is kept on hand for a certain amount of time so it can be recovered should the current copy get lost or damaged.
What I've described are basic backup schemes which will serve most people quite well. You can get far more elaborate (and expensive) should you need more protection due to a larger amount of data or more critical data than most people have or need faster access to the backups.
