The current spec says uniprop returns the narrowest numerical type for numerical properties and boolean for boolean properties, and returns Str for all other properties.

It also mentions a unibool function. At the moment we have uniprop-bool, uniprop-int, uniprop-str in Rakudo, but they are not in the docs or Roast. I think the only useful one of these we might want to keep would be the uniprop-bool function, which could be used to check the absence of a property.
This code below shows an example I think makes logical sense, and is also currently what Rakudo does.

'a'.uniprop-bool('Numerical_Type') #> False
'a'.uniprop-bool('Hangul_Syllable_Type') #> False
'a'.uniprop('Numerical_Type') #> None
'읔'.uniprop('Hangul_Syllable_Type') #> NVT

See discussion here:
https://irclog.perlgeek.de/perl6/2016-12-16#i_13751405
@jnthn