Your consultant is correct. I recommend reading up on the intended purposes of part class and product groups as they have multiple functions within Epicor.
As for financial transactions, to keep this high level, product groups will direct GL transactions for sales and cogs accounts. Part Classes will direct inventory accounts. Take a look at the GL control setups in E10 for Product Group and Part Class, you will see the accounts for each.
As for how you use these fields, it’s up to you and your business needs. There may be other fields you can use to track the data you have been storing in the class and group fields. Maybe add a few UD fields if you can.