The short answer to this question is: no.

Direct (oAuth) access is not more secure than manager (MCC) access.

Regardless of the type of access you chose for your Google Ads account the person or organisation you authorised to access your account has full access to view and modify your data.

The only difference between oAuth and MCC access in regards to the security of your data is that MCC access also enables access to Google Ads UI. However, both access types enable full access to view and modify your data via Google Ads API.