we shall see... they said Harvey was a non-event too in the beginning.
In any case, all this water has to go somewhere!
There are three distinct threats from a hurricane.
The storm surge is the deadliest threat. I live 35 feet above sea level and 10 miles from salt water, so I don't really worry about storm surge. When you see evacuations, the evacuees are typically fleeing from the storm surge.
The second threat is wind. Wind damage is often widespread and can compromise structures. Typically, this means that shingles are blown off roofs allowing rain to seep in a house. Wind is also the major factor causing power outages when it blows tree branches on power lines or knocking down the power lines directly. The thing I fear most from a hurricane is being without power for an extended period of time.
Rain is the final threat, but it usually threatens the smallest number of people. Flood damage is certainly costly, but hurricanes often blow through an area before dumping enough rain to cause floods. Rain is also a more predictable threat than wind. Wind damages structures over a wide area with no discrimination. Flood waters affect homes near drainage arteries (creeks and rivers). If your home is high enough, you probably don't have to worry about floods.
Harvey was far enough from Houston that its winds were a non-issue. But Harvey looped around SE Texas and dumped rain for four days. Rain was the threat from Harvey. And I don't think anyone was under the impression that Harvey's rains wouldn't be an issue. For the overwhelming majority of Houston residents, their houses are high enough that rising waters aren't a danger. City officials prepared Houston residents for 30 inches or rain, which would match the largest floods in recent memory (Tropical Storm Allison). In reality, Harvey dumped up to 50 inches of rain. The people that usually flood had water, but a lot of people who don't normally flood had water, too. But for 98% of Houston residents, Harvey was just an extended summer break. Schools were closed and the city was shut down, but the people were comfortably sitting at home watching TV in air conditioning. They couldn't go anywhere because the roads were flooded, but they weren't in danger themselves.
So to the extent that people thought Harvey wasn't a problem, it was that Harvey wouldn't bring its high winds to a densely populated area. Remember, the wind threat is the one that is widespread. The risk of flooding was known, but even with 50 inches of in four days, 98% of Houston homes didn't get any water in them. And of the people that flooded, it wasn't the first time for many of them.
Don't say that Harvey caught Houston by surprise.