Is Acronis True Image encryption secure?

Jan Doggen is correct - The 'worth' of the encryption in this case (which has nothing to do with the actual encryption as I'll explain below) is completely opinion-based, so you better rephrase that. But I'll try to explain the meaning of the EULA:

This means that they have the symmetric AES key to do the decryption. Their encryption isn't worth nothing per se, it is worth as much as you are willing to trust Acronis. Meaning, the AES encryption is in place, but they have the ability to decrypt your data. The question is do they? or will they? These are questions no one on this site will be able to answer. And the decision of trusting them is yours to make.

Acronis True Image lets you encrypt client-side, too. The password you set creates a private key that only you know. So, if you set that up, Acronis won't be able to decrypt your data even under a court order. To set a private key, from the desktop tool "backup" pane, go to "option" > "advanced." Set your password under the "backup protection" heading.

If you don't set private encryption, the company still encrypts your data at rest, but they have the encryption key.