Salesforce CPQ differences between Lightning Experience and Classic UI

Using the New button on the quote related list from the opportunity record does not pre-populate the Opportunity lookup field on the new quote. The workarounds are to either create a workflow rule, process builder action, or create a custom New Quote Quick Action to populate the opportunity lookup field.
When a user creates a new quote, clicking Save does not take the user to the Edit Lines visualforce screen in the quote line editor. Instead you must navigate to the newly created quote and click Edit Lines.
In Lightning communities the Salesforce CPQ New Quote action is not available on the Quote related list or the Quote list view unless a custom Lightning Component is created and set as the 'Lightning Experience Override'. In order for users to create new quotes, custom New Quote Quick Action or Global Action can be created to Allow Customer Community Users to Buy Net-New Products .

When using Salesforce CPQ to Generate and Send a CPQ Quote Document, the Save & Email button saves the Quote Document, but opens a new browser tab in the Classic UI that remains open rather than closing after sending the email and returning to the Lightning UI.
Lightning Community Partner Users see an Invalid Page Error when attempting to click Save & Email from the generate document page or the Email action from the Quote Document record. When admins use Save & Email the email pop-up successfully appears. However if a non-admin tries to use the Email functionality, the pop-up window directs to a url that includes a '/s/' which causes an invalid page error.

In Lightning, the Create Order button will not create an Order with Order Products from an Opportunity with a Primary Quote. Use the Ordered checkbox from the Details tab on the opportunity or quote to create an order with order products.
In Lightning, the order status picklist will display all status values, whereas in Classic you will only see the relevant status values to the order category (draft/activated). This also applies to the visual representation of the order's status path, where users can select "Mark Status as Complete".

NOTE: Once an order is contracted your order status needs to be an activated category. Please consider the sequence of the 'activated' category if modifying the Order Status picklist fields and ensure that your path includes 'Activated' categories after 'Draft' categories.

In Lightning, the Clone with Related button copies the record then opens it for editing. In the Classic UI, Clone with Related opens the record for editing with the original's values pre-filled. It inserts the record only when you hit save.
In Lightning, the Clone with Related button will not clone a custom field value on a related object. The workaround is to use the SBQQ__CloneWithRelatedClassic button in the Classic UI.