In some cases maybe, but hopefully most founders who obtain some personal liquidity in later rounds are not sadistic/hypocritical enough to deny their employees the same opportunity.

But you're right, one potential large risk is a Chris Sacca -like situation, where one investor/investment group uses many anonymous buying agents to acquire a huge stake in a takeout/IPO candidate, via secondary liquidity. That can mess up a final outcome for whoever thought they had control over the cap table.