It's not the 50-50 split that's bad, it's the part where you don't talk about vesting. I have a friend who confounded a startup with another guy. It started with my friend agreeing to put in $20K and some advice, the other guy working full time, for a 50-50 split. After one year my friend has put in $200k+, worked full time building business, marketing, etc, while the other founder does his own job poorly.
They agree my friend deserves more equity, but can't agree between a 70-30 or 80-20 split, so other founder hires lawyer and holds my friend hostage for a lucrative buyout. Company is now worth significant amount of money almost solely to my friends hard work/investment, but legally he can't move forward until co-founder signs off on every decision.