Most often I see bad performance and lack of attention to detail. Most parser's are not able to properly read field and place the information in the proper corresponding field.
Thanks Ryan, good feedback . Iam sure people here has lot to share about their experience.
Vinay - I just noticed you work for one of the top parsing services, so I'm not sure if you question is a lark or serious.
If you could use that mythical semantic technology I've been reading about for the last 10 years. Something like this very simple and cheap API that can be implemented and integrated very easily.