One of DOD's crucial strategies is using the environment in your favor to help you towards victory. Sadly, some rules do not apply to all the dragons. Objects/bodies with collision can block projectiles, such as acid spits and plasma shots, which means you can use trees and other dragons to protect yourself from being hit. The thing is: fire does not follow that rule. You can hide behind the thickest tree in redwoods, or a huge rock, or maybe even tuck yourself inside a log and a Flame Stalker can still burn you from by breathing towards you from the other side of the "object".
An Acid Spitter and a Shadow Scale need to turn around said object to be able to hit the dragon behind it, while the Flame Stalker is guaranteed to put you on fire.
The same applies to dragons' bodies, which don't present collision (you can walk through them like they're a ghost) unless we're talking projectiles.
Let's think about a bite fight with ASDs around. An ASD shooting from the distance has a high chance of hitting the FSs from their group while trying to aim for the enemies (with so many bodies spinning around) while one FS can flame 360º and effortlessly flame every single dragon around.
A Flame Stalker can protect another FS standing over it, serving as a body shield against SSs trying to pelt it from above, while, for example, an ASD cannot hide itself under an FS and protect itself from the flames 'cause, unlike with projectiles, the fire phases through any body.
Flame Stalkers are already the hardest dragon to kill, and the unfairness of it's fire damaging through objects makes combat unbalanced.