It needs to be big enough to hit 10 blocks a day minimum to offer okay proctection. against variance
a large pplns could have a bad week. and getting 0.003 a day for a week in a row can happen
you would be down 0.021 and the pool shuts down
So pool folding and you getting a big loss the risk is worse with pplns.
Frankly a large pps and or a large pplns folding has not happened in a long time.
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A PPS pool is affected by variance, not the miners, the owners of the pool.
Since the owners must pay that variance to keep a smooth lower reward for the miners, their wallet must always be big enough to cover paying the down turns.
So a PPS pool may well fold due to expected mining variance.
I'd guess that the most recent example of this would be NovaBlock, however a large pool bought them out so hopefully no miners lost out.
A PPLNS pool is affected by variance, yes the miners, not the owners of the pool.
So the owners wont go broke, and the miners wont go broke either, they will simply have a down turn in rewards, just like they have up turns in rewards also.
No one is forking out of their BTC wallet: the value of the down turn, the miners will expect over time to level out, as said, they have ups and downs.
The size of the pool will decide how long this is, but even over 250 blocks this should be pretty minimal.
During a 5% drop in luck for a PPLNS pool, everyone mining is receiving a 5% lower reward.
During a 5% drop in luck for a PPS pool, the pool is paying everyone that 5% difference.
This is why it matters if the pool has a large enough wallet, and can keep that wallet balance large enough,
... and it is not a small amount of BTC.