Zillow has grown rapidly and seems to control mindspace. However the technology behind them is not tricky and many competitors are to be expected. Those with the marketing budget should win.
Zillow goes up and down with the real estate market. Markets do crash and burn.
Zillow has made some recent deals which do not seem wise. The NYC market which should be consistently lucrative has become controversial with a recent deal with the largest broker. Without Zillow, Douglas Elliman would sell the same amount of Real Estate in New York. But without Douglas Elliman, Zillow would be irrelevant in New York.
Several other lawsuits are nipping at the heels. The real estate business is litigation prone at best of times. As you become more important you get sued.
George Gutowski writes from a caveat emptor perspective.