the main one is that most people don't own the model. so if you use openai / anthropic / etc then you can't use token masking. in that case, reprompting is pretty much the only option
You're right, though, that reprompting works with pretty much everything out there, including hosted models that don't have tool use as part of their API. And its simple too, you don't even need to know what "token masking" is.
Reprompting can also apply arbitrarily criteria that are more complex than just a json schema. You ask it to choose an excerpt of a document and the string it returns isn't an excerpt? Just reprompt.