Some cons:

• You're polluting namespaces with constants
• You can only express boolean conditions without calculating them
• Every data structure has to have a slot for all possible conditions
• Any code that deals with conditions has to know about all possible conditions
• You're separating the meaning of a thing from the description of the thing
• You're requiring *more* syntax, and it's punctuation
• You're dictating the implementation of future potential data structures

They're a tool, like everything else, and have their uses in certain circumstances.