It'll be very interesting to see the IPO valuation - I bet that more than a few investors are going to be disappointed with their returns. Does anyone know why

It'll be very interesting to see the IPO valuation - I bet that more than a few investors are going to be disappointed with their returns. Does anyone know why Goldman seems to be the bank of choice for the Unicorns? I don't know how IPOs work but Goldman has underwritten or advised on Snap, Okta, and now Dropbox. It seems like there should be smaller banks that could do the same work for less money, but perhaps I'm discounting the value of Goldman's contact list.