回复：回复：给你举几个例子。

一个用NIW申请绿卡的人，至少要有PHD吧，这样的人怎么会非法打工或有犯罪记录呢？
Why not? You think PhDs can't have criminal record? and won't 非法打工? For example, if you have H1 and you do consulting on the side, then your payment could be ilegal if you don't have another H1 with that employer. Or if you ever worked off campus when you were F1.

我想这种情况至少不会很多，
You are asking for reasons for denial, not the most common reasons.

另外，如何理解 更换雇主之间有间隔会失去合法身份呢？
Easy. Were you ever laid off? If it took you 4-5 months to find new employment, then for that 4-5 months, you were technically out of status.