It's possible in SQL to write complex ON section. Something like this

SELECT * FROM a
JOIN b ON a.id = b.id
    AND (func1(a.x, a.y, a.z) OR func2(a.x, b.x) OR ...)

It's OK cause it has equality a.id = b.id and we could make it via equi-join. But we should move complex (func1(a.x, a.y, a.z) OR func2(a.x, b.x) OR ...) to WHERE or support complex filters in JOIN.