They do not get staggered like thralls. No matter if they are against many, they will attack nonetheless.
But since you cannot “armor” them like thralls, they are weaker. A lvl 2 purge can crash even a lvl 20 Dalinsia, so loosing a lvl 13 sabertooth is almost natural.
Pets however can go really high on vitality, higher than thralls.
So they can reach until purge lvl 7.
Before the update, the colossal snake was simply magnificent.
The most unfair pet nerf however went to wolves.