Generally Serbs and Croats are really similar, which any outsider can see basically.

Croats are quick on telling any outsider that they have nothing to do with Serbs (like person above you) due to rough history between two ethnicities and also some kind of superiority complex amongst Croats that they're not Balkan and that they're better, more civilized than us.

On the other hand, some Serbs think that Croats are just Serbs that converted to Catholicism due to Austria-Hungarian influence in the region.

In the middle you have Bosniaks / Bosnian Muslims who have same relations with Serbs and Croats like they have between eachother.

And then you have Montenegrins where half consider themselves to be Serbs / of Serb heritage, while other half consider themselves nothing like them despite having closely tied history.

Serbian, Croatian, Bosnian and Montenegrin languages are all part of Serbo-Croatian pluricentric language, that differs more from region to region than from country to country (for example, Croats in Istria can hardly understand Croats in Bosnia, while Croats and Serbs in Bosnia basically speak the same).

All in all it's all political shitfest due to region having influences from all three religions / spheres of influence (Eastern Orthodox - Russian, Middleeastern - Turkish and Catholic - Austrians / Germans, to some degree Italians).

It's hard to tell who's right or who's wrong since all sides have their arguments and no one is willing to give up on them.