All right, so are you upset about the (apparent) fact that the bonus exists, or that you weren't told about it?
I'm pretty sure that since the dawn of time, people have been saying that spearmen should have no chance against guns.
My complaint would be (inspired solely by the OP's example) that the difference in unit power should be narrowed a bit if there's an era bonus, because the two together make for overpowering situations that remove all hope from catching up from a tech deficit if your neighbor should decide to whale on you when they get it. (Haven't played the game yet.)