I haven't had those problems with poloniex and I don't do margin trading, but it does bother me that the bitcoin price is so high relative to other exchanges. It is apparently due to an issue with USDT which means it could be risky to trade with USDT on their site.
They even delisted some alt-coins without any explanation whatsoever. Personally, I haven't had any issue's with Poloniex so I guess I'm still fine doing trading with them. But the concern raise by the OP looks fascinating, it this is a valid allegations, then something must be wrong with Poloniex. If you have more proof then show's us maybe someone will chime in and collaborate your claims.