Criteria-based sharing rules allow for automatic sharing of records based on specific criteria, such as product line in this case. By setting up opportunity sharing rules based on the product line, each product engineer can be granted Read-Only access to opportunities that match their assigned product line, ensuring they have visibility into relevant opportunities without manual updates or intervention.

Default Account Teams and Default Opportunity Teams are used to automatically assign predefined teams to records but do not inherently allow for automated sharing based on specific criteria like product lines. These options would also require manual updates to ensure that the correct engineers are assigned, which is less efficient than using criteria-based sharing.

Salesforce Documentation References:

Set Up Opportunity Sharing Rules

Opportunity Sharing and Criteria-Based Sharing Rules