The disadvantage of being a person who is too egoistic is that you will lose many people who will sympathize with you, lose respect from others, are considered to have a bad personality, even no one wants to have a social relationship with you which will certainly have a negative impact on your life in the future.
I have personally witnessed this. Nobody understands that you are acting egoistically because of your inherent character or maybe that's your normal reaction to a situation. People always think of you as a bad person and break ties with you when they see you acting egositic.