My perception is that one drawback of earnestness is that it often comes off as just being boring. The most earnest people I know are not popular. The most popular people I know are more like twinkle-in-the-eye-kind-of-BS-but-not-pathological-liars. I don't mean to say "grr, why aren't nerds more popular!". I mean more that this specific kind of earnestness seems to preclude the playing of certain conversational game…
Still, I have learned that being earnest and being fun are not opposites. Sometimes earnest people are spending most of their time focused on work (and if they are good natured, its fun to work with them!). But one can get better at being fun if you value and prioritize having fun with other people for its own sake.
I have also found that folks (including me) particularly enjoy friends who are earnest about their work / side projects. I especially enjoy spending time with friends who are earnestly interested in areas I know little about. Their energy and enthusiasm are infectious and talking to them about their area opens these clear windows into new worlds that are otherwise hard to find.