There appears to be some confusion in the community between the difference of nice guys and pushovers. Let me see if I can shed some light on the matter.
Nice guy: someone who has a positive attitude but is realistic in life; someone who doesn't put people on a pedestal and refuses to be bullied or threatened but yet treats them with respect; someone who's willing to help another person out every now and then but refuses to be guilt-tripped to help someone every single minute when he has his own priorities. In other words, a NICE guy.
Pushover: someone who does a favor for someone without a return in that favor and never seems to want a favor of their own; someone who idolizes another because they are so in awe of the person's humane traits and yet doesn't even treat themselves with respect; someone who has a fake positive attitude all the while having a very messed up mindset and lives in la la land. In other words, a PUSHOVER.
Now, some guys verbally state that they are a nice guy. But whether they are is up to you to find out, if you choose to. Whether they are telling the truth, lying or just flat out wrong depends on the individual guy. Typically, they are lying. But not always. There are certain people in the community who like to test others to see how far (or short) they would go. Only you can determine what your preference is. Not everyone's preference is necessarily the same. Some people tend to (possibly like to) mix nice guys with pushovers, for different reasons. Whether those reasons are valid depends on the individual person. It's possible that turning nice guy into a derogatory term is their way of them turning into a bitch, which is equivalent to a guy who's an asshole that plays women.