Hi Vladimir,
You are both right and wrong.
First, 'brittle' [as well as ductile] is no a technically defined term but a
descriptive comparative term; thus something can only be more or less brittle
than something else.
Second, these LF-solders have a higher tensile strength and a somewhat higher
modulus [at the same temperatures and strain rates, of course], as compared
to SnPb; this results in higher stress levels on the interfaces under the same
loading conditions and thus interface failure woulkd occur earlier for
Lf-solder joints, whether the interfaces are more brittle or not.

Werner

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