Besides the unique appearance and rarity the Unicorn Chomper is special in that the Burrow ability is replaced by a Rainbow Warp. The Rainbow Warp allows you instantaneous movement, like the Scientists Warp ability, so instead of being able to see the Burrowed Chomper coming at you there is a whinny, a burst of rainbow and suddenly he's right on you- a skilled player can put himself pretty much directly behind a Zombie almost instantly, Goop and go for the Chomp before you even properly start to turn around.
Whether or not he's "good" or not depends on how he gets played, whether you're comfortable with him and what you expect from your Chomper.
I haven't even began working on getting to rank 313 on GW1. I'm never gonna have him...
While I was Mastering my Unicorn Chomper I consistently undershot or way overshot my target with the Rainbow Warp, and the fact that you need pretty level, open terrain for it to work properly tripped me up uncounted times but eventually you start to be aware of whether or not an upward slope will block the charge forward and I got pretty good at judging the distance for a good 'right-behind-you' Warp or in aiming myself at a terrain feature which would stop you where you anted to be.
Without the Burrow you need a whole new level of precision to get a good Vanquish count.
