Some websites say that the species you choose gives you an in-game bonus to your character's stats and such, other websites say the only difference is aesthetics and social abilities. Which one is true?

It's only looks and social. All stats across the board are the same.

all the races get a 'novelty' ability... They don't really have much of an impact, the only one I know of that sounds 'usefull' in any way is the Miraluka one, you can detect the targets alignment. Others are just fun to use at times

 

From what I understand, people complained about the 'unfair' advantage some species got over cirtain classes so the bonuses were removed before the launch of the game