Frequently who can spam so much burn can do it using a lot of skills, wich make them vulnerable and they're damage poor if you clean the burn stacks or have resistance. An example is the Guardian: you can spam your skills and inflict more than 18 stacks of burn, but if the enemy clean it or have reaistance you lose almost all your dps for a lot of time.
Burn is strong, but is (frequently) a conditions hard to stack or can't be spammed all the time like a lot of other conditions.
In 1v1 you can control the enemy stacks using clean skills and fighting the enemy wisely.
In a 3v3-4v4, if you have a single class that spam clean (elementalist, for example), you can ignore the burn damage at all because it can be removed easy spamming heals.
The classes and builds able to spam it can't inflict so much using other conditions or have different cooldown to keep an eye on, also using a lot of skills, making them weaker in defence or damage if you survive the "burn burst", fighting easy against them if you use your skills wisely.
Burn is really strong, but you can counter it easy timing your skills
