Is cloud storage safe to use?
-techidave
Is storing your files on other people's computers safe to use?
-Tuxman
We live in a universe where one is constantly weighing risks and choosing the best imperfect solution.
No action is risk free.
The online backup tools use encryption to store your files on their computers; they encryption is performed locally and the online service does not have your encryption key, so neither they nor anyone else but you should be able to decryption the contents they store.
You have to balance the risk of something going wrong with that encryption scheme against the risk associated with being burgled, caught in a fire, having a hard drive crash, getting hit by ransomware, etc. Pick your poison.