If you use a reputable, reliable provider, your files are extremely secure. Be sure to choose a service that encrypts the data not only as it's being transferred but also as it's stored in their cloud. You should also check to see how your encryption key is stored; if you've got serious concerns, pick a service that allows you to create your own private key (and don't lose it, because the service won't be able to recover your data if you don't have the key!). Finally, it's worth asking where your provider's data centers are located; ones in the EU and US are preferred.