Well there are issues with the POW Model regarding its effects on ecology and network security but if NEM sees a stronger adoption we will see its impacts with new nodes using up resources and some clever hackers might find security issues too.
Leads me to another question :
How is POS/POI not more centralized ? Give 156 Stakeholders a Stake for small money instead of giving people the chance to get them in a continous period ? Even if the stake to claim with PoW gets smaller over time isn't this a fairer approach even if you want to reward the risk takers ? I am sure there were more people grabbing a Stake in Bitcoin early than in NEM.
There are a few more stakeholders than that, the most conservative number is at least 1000 unique users i think (even with all the sock-puppet controversy). Read the population statistics. The theory is that after the initial distribution, new users can continuously come in via exchanges and multipool. It is actually very similar to POW cost-wise but without the waste energy.