No, not every system is hoppable. Normal PPLNS isn't if you do it right. It's just time based PPLNH is.
the "normal" PPLNS you are talking about is a bit more complex then the PPLNS most implement. Do you know any pool that implements PPLNS correctly so no differencechanges can be hopped? But still nevertheless, the difficultychanges are only limited...
PPLNH is a drawback for your miners if the pool hashrate drops.
This is a huge increase in variance for your miners. Variance doesn't change with hashrate under PPLNS. As your miners see their variance increase, they'll leave since the whole reason to be in a pool is to decrease variance away from solo mining variance, not tend toward it.
Variance does change with hashrate in PPLNS, if N is small enough. The only thing that is true is that people not loyal to the pool will not get any money if they poolhop due to the fact that the rounds are often > 24h, but this is also true for about 50% of PPLNS, N=1/2D. So yes, in the end, non-loyal users are punished the most, poolhoppers will not get any pay when the round is >24h. But those loyal, which are the miners that mine every day, will make up for that later on.
So basically what you are saying is that the variance is higher then for example a PPS. This is true. This is true for all PPLNS systems. But loyal users - users that mine every day, will not notice, as they meet the requirement to always be inside the payment window. And this is also fair. This pool gives you some variance on the pools luck. with PPS you don't have any variance, with PPLNS systems you will earn more if the pool is lucky. This is not a disadvantage... If the pools lucky, you get your share.
I still fail to see any reason why this system would be cheat-able (getting - statistically proven - more earnings by hopping the pool) except for the difficulty hopping - which I still haven't seen that it is just more then a theoretical remark that is practically useless.
I really appreciate everybody's thoughts about this, and I welcome all feedback. If someone can point out any way to hop the pool, in a way that can be both statistically proven and relevant (the variance of the pool due to the PPLNS nature will be greater at this moment then the possible rewards you can get from difficulty hopping, so this is irrelevant as it is practically unfeasible), I will change the payout system again, so the pool cannot be cheated.