We demoed V3 on and off for at least a full year and always reverted back to V2 due to a lot of impact to our workflow. Eventually we tried it again and then got trapped in V3.
For one thing, every function seems to hang and make us wait when that never happened on V2.
When you combine orders on V2, the item totals combine as well as the customer's shipping paid combines. That makes it easier to set value for insurance purposes.
Orders will also occasionally get assigned to a batch number that we never created.
There are others but we've been begging to get back on to V2 and I can't get any satisfaction in that department.