I've trained for spring ultras (50k,50m,100m) in the winter with a substantial amount of mileage on the treadmill.
Pros:
- It makes logistics easier since you can have water/food lined up at the ready and a bathroom break nearby.
- Inclines/slight declines at the push of a button. This is especially helpful for trail ultras, although most treadmills only go to 15% and you can get steeper for some ultras.
- Tempo/LT runs at the push of a button. Keeps you at a steady, tough pace, just go along for the ride.
Cons:
- If your ultra is trail then you wont develop your "trail legs." You need to be able to have a constantly changing stride, especially if your ultra has rocky or challenging terrain.
- It's best to practice logisitics (carrying water/gels/food; dealing with inclement weather, etc.)
- Lack of wind resistance