How hard is it to implement a backdoor in a fully opensource project? (Assuming the project in question has a lot more eyes on it then an overworked developer)

I2P?

Not Monero!

I use it to buy gift cards in India, currently the most populous country on the planet(for better or for worse), that adds up to... 35-45% of the world population; the figure does not even include the African and south American countries where there is a significant adoption of cryptocurrencies due to high inflation rates of the local currency.