"A spike in hashpower will get your pool banned by other nodes on the network"
How sensitive is this? If someone with, say, 10Gh decided they wanted to mine PHC and joined the pool, are you saying that, just the act of joining the pool will bring down the pool?
Yes, large miners can bring down the pool if the hash-rate of the network is distributed evenly among all peers.
It has been set to be not too sensitive and can be adjusted. Peer block heights are also evaluated with an adjustable tolerance. For the past 24 hours, 1 seed node did not ban users but disconnected them (for testing purposes)
For example:
If the network hash-rate is 100 GH and you join the pool with 10 GH and start generating blocks at 10% what the rest of the network is mining then you're within the "safe" limits. But if you join a pool with 100 GH or more, you will most likely cause that pool or if you're solo-mining (your wallet) to be banned for 24 hours. 50 GH would be getting close to "un-safe" limits if 100 GH of the network was being mined by several peers/pools and not just 1.