I know in GW1 soldiers hiding on all the rooftops was a common sight, mostly to avoid Chompers of course. I see that less in GW2 likely mostly from the different maps.
Burrowed Chompers are only vulnerable to Engineer sonic grenades/mines or when they begin their burrow there's a window where you can knock them out of it with All-Star charges (Super Brainz kicks used to do this too but not any more after the latest balance patch). With more classes there are generally less engineers so less zombies capable of stopping their teammates and themselves getting chomped. The ability to burrow up walls would be extremely game changing, making Chompers too good for sure.
It doesn't help that the sonic stun doesn't always stop the Chompers getting you anyway. I've seen it often enough in matches; Chomper burrows under a mine, the mine goes off but the Chomper still chomps its victim. Never good when your countermeasure fails even though it supposedly worked.