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.
First, where did you get the 156 number from?
Second, what makes you think that Bitcoin is ever so secure and not subject to cabal attacks? I hope you do realise that miners in bitcoin have reached asymptotic levels and in fact is on the decline. Have a look at this. The tipping point
WILL happen with the passage of time.
http://www.coindesk.com/51-attacks-real-threat-bitcoin/I need not say more. Good luck.