This is what I found about undercoating:

Pro -
1. The benefit touted by most "old timers" was the reduced noise and vibration coming from the vehicles structure. It acted as a huge noise dampening blanket under the Vehicle. Chassis' are much tighter than decades ago I don't think today's modern vehicles benefit enough from undercoating to warrant the cost (or risk).
2. Easy enough to do it yourself.
3. It's repairable; since it bonds to itself... just touch it up with more liner.
4. Looks professional in appearance.

Con -
1. Undercoating is not the same as rust proofing. It is a coating resembling spray-in bed liner material (but I'm sure not as durable). The common "myth" was that it prevented rust, but in many cases, because of the myriad of dangling pieces in a chassis, complete coverage is not possible, thus resulting in "trapping" moisture. Undercoating was also much less resistant to tearing and scraping than bed liners, which allowed additional moisture into the chassis.
2. May tend to get tacky (gooey) in the summer's heat.
3. Salt will collect in small pockets or chipped off areas of undercoating and will trap salt. It is more harmful to undercoat than it is to leave the metal exposed. The undercarriage liner may hide rust spots from sight.
4. Rust prevention is as easy as taking your vehicle to the wash to spray salt off undercarriage after a romp on the beach.