Just in addition i would like to share my scenario solved with @Prakash Kumar hint.

My call at 'customer.comment' was returning a string "car, roads, breaks, lights, tires, gears"... The magic split method "inherited" from web.assets_backend convert it to an Array / List and our pretty good <t t-foreach /> works properly.

<t t-if="customer.comment"> <!-- This means, if there are some value at this call -->
    <t t-set='prod_serv' t-value="customer.comment.split(',')"/>
        <t t-foreach='prod_serv' t-as='ps'>
            <li><t t-esc='ps'/></li>
        </t>
</t>