I really strongly prefer #3 (Floating Mountain). The flat aesthetic simply makes sense given the design direction within the tech industry over the last few years (and in fact the illustration could be cleaned up to be flatter, with no thin-line borders). The connection with the mountain and the title is obvious but effective, and the fact that it's floating is a visual allusion to a cloud and references the loftiness of tech valuations and the difficultly of scaling.
It just looks cool - I also think that photo covers, if there's no visual originality to it, looks cheap.
Unscalable uncovers Silicon Valley’s lesser-known backstories of failure and achievement. Interviews (GitHub, Zenefits, DoorDash and more) showcase eclectic strategies and experiments, revealing the long and unpredictable road to startup success.
It just looks cool - I also think that photo covers, if there's no visual originality to it, looks cheap.