A lift cannot void the whole warranty. Millions of new trucks are lifted with no problems. In fact numerous dealers will sell you the lift for a new truck. Of course if there is an individual part that is adversely affected by the lift then you will pay for that part.
The stock transit sits very low so the main benefit is better approach, break-over and departure angles. The ground clearance is still limited by the diff and POS lower rear shock mounts. I will be changing the rear shock mounts as soon as I can.
This is not going to turn your Transit into a rock crawler but you should be able to drive fire roads and dirt roads without dragging the rear at ever ditch.
2017 Ingot Silver, Cargo, Medium roof, 148"WB, standard length. 3.5L Ecoboost.