In all honesty you shouldn't trust any of the exchanges. I stay as liquid and spread out as possible. My view, however, poloniex doesn't want to list what i refer to as shit coins because they impost a great risk to the exchange.
Since the market is new, I doubt there is any insurance. I think poloniex decision to lock certain coins is a form of consumer protection. Market share at poloniex speaks volumes. The most important thing is to stay liquid.
I will never use polo again. As crypto goes more main stream they will be forced to clean up their act or go out of business. At present I have no faith in them as a business. They basically freeze funds with no explanation and refuse to resolve support tickets for weeks at a time! With the losses on crypto exchanges over the last few years I do not trust them. There are several alternatives which offer a much better service so I will give them my business!