Its all about the application, nothing is really 'BAD' as such it just might not be effective as a whole against something else. But like someone said up the top, its that they train to fight the one style more or less or are suited to a form of fighting.
In my opinion its not underestimated, but the people that use it are in comparison to other more well known arts. Any technique from any style can be made as effective as any other if it can be applied correctly, no matter how crazy it is if the person doing it is good enough and well versed enough they can make great use of it. Within reason.. its all to do with the ability of the fighter using said technique, not that anything is 'crap' as such, Just that the fighter might be awful at everything else and has no fighting mind to adapt his techniques to the situations as they come in a fight.