It's worth noting that the qualification should be "easier." But the POW is only one side of the DoS problem bitcoin's model can be forced to experience.

By carefully attacking the network links between miners, someone can significantly extend the amount of time that an attack can take place in. These attacks are expensive, but orthogonal to any PoW algorithm.

Then the problem transforms into: "can I trigger enough small pluralities of miners to split and form opposing but equally long viewpoints?" The only resolution to such a scenairo is to discard most of the divergent blockchains arbitrarily by human consensus.