I definitely have had success with a brother's Y-DNA test.
Our family goes by surname "A", but my papaw always told us, "we should have been "B". Papaw knew his father was born outside of marriage to "A" female and "B" male, and had gone by his mother's surname. After many years of struggling to document this A to B relationship, I finally found paper documentation as well as "B" surname cousins who knew of the relationship and were able to confirm it with even more documentation.
25+ years of research up that "B" line left me hitting a brick wall at my 4th Gr-Grandpa, so a brother did the Y-DNA test to help me out. We had some family lore that this 4G-Grandpa was supposedly "orphaned" and was raised by surname "C". When the DNA test came back....we were actually matching surname "C". Two other of our "B" surname cousins on different down-lines from mine did testing as well and matched my brother..all of us matching surname "C".
The DNA shows that my 4G-Grandpa "B" was born outside of marriage to "C" male and "B" female, and had gone by his mother's surname. Land records confirmed that surname "B" and surname "C" shared an adjacent property line for many years, and then county court orders, deeds, and other docs confirmed that these two families had always lived within very close proximity of each other even during their migration from western Maryland to southwestern Virginia.
Without the Y-DNA test, I may never have found my true "C" paternal lineage.