[EricH]

Cons: Weight: There aren't a lot of things heavier than the compressor in the kit, and the tanks weigh a bit too.
Limited use: You can only use them for a few things. the last time we used pneumatics, we had: 2 gear shifters, a winch lock (to keep us on the bar in '04), a wheelie bar deployer (to keep us from tipping), and an arm extender that didn't work too well against gravity.
Battery drain: If you use them a lot, the compressor will reduce your battery power as it runs.

Pros: Simple: one shot power for just hooking it up right, and a minimun of programming.
Won't stop when power turns off: useful if you need to climb on a bar or retract something at the very end.
Hard to backdrive: won't reverse without instruction, as long as the tanks are full.

As a side note, use of pneumatics is a lot better than it was back in the day. Many years ago, you got one huge tank (13 lbs or so, 10% of your weight budget) that would store all your air for the whole match and one cylinder that was your entire system (and often was the wrong size). Now it's a compressor, storage tanks, and your choice of cylinders.