The proper selection of coil is key as to how the shock performs. They are rated in #’s such as 350 and 400 #’s, for instance. Too heavy and harsh ride, too light and easy bottom out. Bottom out and you risk breaking the shock mount on a frame.
Air, typically easy to adjust into the ballpark that it needs to be in. With correct valving, they can be either firm or soft and plush as a 1970 Cadillac.
Coils tend to perform very well on washboard since they can rebound fairly fast and not pack up.
Air. While they perform very well, care in setting rebound fairly fast as well as air pressure for the riding style. Slow rebound causes pack up and loss of suspension function.
Pack up is when the shock travels part way but cannot rebound or cannot do so fast enough to be active and plush.
Push Industries specializes in shock modifications to improve performance and should have some write ups on their site. There are a few other operations that do shock services as well.