Maybe I'll regret this, but here it goes...
I don't like Carlos. I don't know him in real life. He may be a perfectly nice person, but I do not care for the Carlos that has been portrayed on the show. It has nothing to do with his hair, accent, heritage, etc. I do not care for him because his general cooking knowledge appears to be less than the other chefs. He strikes me as unprepared when he shows up with knives that are not appropriately sharp. He seems hypocritical when he complains about someone not helping him, and then turns around and says "this is a competition" when he is expected to help someone else.
These are my impressions from the totality of what I have seen on the show. I do not go back and re-watch episodes even once, so I certainly do not dissect them for detail. I do not have any desire to pick apart each instance of behavior and document it here. Impressions like this can just form over time. They don't have to make sense to other viewers.
I like Nina, but I have gathered that there are many here who don't like her at all. I wouldn't argue with them about the reasons they don't like her. They are entitled to their opinion.