The most important thing for any Point of Sale software is that it needs to be lightening fast. The product list can be downloaded to local storage for fast searching and filtering, but the cart also needs to respond as quick. Items should appear in the cart immediately and the totals should be instant. Any POS which requires a call to the server is going to seem slow and clunky, and a cart that hangs mid-sale is useless.
One concern is how to deal with any sync issues at checkout. Having the POS as dumb terminal means WooCommerce is responsible for calculating the correct totals, tax, stock management etc etc. Moving the cart logic client side means the POS is responsible for getting it right.