There is always limitations.
SynFacilSyn is very flexible, but has no the flexibility of a regex lexer.
It's not possible to define dynamic delimiters. But the syntax can be defined by code too. So it would be possible to modify the language definition while writing, but I suspect It would be some slow, because SynFacilSyn has been designed for to be a fast lexer, not for to be fast on re-defining the language. I haven't done such experiment.
There is some features, I have contemplated for future versions, and I have analyzed some kind of dynamic definitions, in order to deal with the HTML/XML format.