Note that all XQuery Full Text queries "work" in BaseX,

No, sorry. I mentioned a query that doesn't, with or without index:
//SPEECH[ . contains text "romeo juliet" all words]
It *should* return 42 items on the whole collection of 37 plays,
and it returns *nothing*. And no relation with the way BaseX chops words.

but not all of them take advantage of the optional full-text index. The reason is
that we initially put most effort on a 100% compliance with the XQFT
specification – and, to the best of our knowledge, we are still the
only implementation that complies 100% with the specs

With the specs, fine, but with the tests? ;-)

implementations are coming closer, though) – and we are gradually
increasing the number of XQuery expressions that are recognized by the
query optimizer.

Perfekt.

By default, whitespace nodes are chopped by the BaseX XML parser;
that's why snippets like...

<SPEAKER>ROMEO</SPEAKER><LINE>Is the day so young?</LINE>

..are tokenized to "romeois", "the", "day", etc. This may look pretty
weird, but it makes sense if you look at examples like..

"<b>T</b>his is funny" contains text "This is funny"

Well this is funny indeed. If I am not mistaken, that means that BaseX would find "This" in the 2nd example but not "Romeo" in the first example.
I guess that words crossing an element tag is something very rare.
So in other terms BaseX works well in a very uncommon situation, but fails in much more likely cases... Well, it is your business.

Sorry if I am a bit ironical. BaseX is still an impressive product in quite a few aspects. But I think I will go on with Qizx for a while.

Regards