Perspective is important. In moderate rain most good boots will keep you dry, but any boot can be "overwhelmed" in the right situation.
To wit: for a while four of us locally owned DR650's and we regularly rode the fire-roads in the nearby national forest. We would set a date and then ride rain or shine.
One of the guys wears only the best gear and his BMW gore-tex lined boots had always kept him dry.
But one memorable rainy day he went down in a deep mudhole. I was behind him and it unfolded as if in slow motion. He and the bike started down.
The bike disappeared under the water so that only the right handlebar tip was sticking up above the surface.
Then he slipped and fell and half-disappeared into the same water hole.
I skidded to a stop behind him and parked my bike and waded into the hole and grabbed his bike and stood it up (based on the theory that if he could get his feet under him he could stand up of his own volition). Hey, what are friends for?
Needless to say his gore-tex lined boots failed the test. They filled up with water, as did my waterproofed and usually "keeps me dry" work boots. We both sloshed water the rest of the day.
Later that year I bought a pair of Aerostich Combat Touring Boots and no sooner than I unpacked the box I applied a liberal coating of Aquaseal.
So far my feet have never gotten wet in the CTB's. But the boots are 14" high - I suspect that in 15" of water I would be in trouble!