we are looking integrating to NetSuite from the orders object in Salesforce. What are your opinions of using orders instead of the opp in Salesforce?

Use any middleware for the integration else with the help of out of the box functionality create a scripts inside netsuite and Salesforce as well

Without SF Billing, we sent SF Orders to NS Sales Orders and NS Return Authorizations. It worked well... we reference the Opp ID in NS