Look at Shen's old W, Feint. It shielded him for about 200 damage at max rank only on himself. Now, he has more of a team oriented ability which can block auto's of any damage (imagine blocking a Vayne's true damage, Jhin's 800+ damage autos and a Yasuo crit for everyone in the circle, at the same time.
It honestly does almost the same thing by blocking damage, but it's better for your team or in late game scenarios where carries do massive damage with their autos. Sure, there are drawbacks, but all skills have drawbacks.
I'm a Shen main, and I honestly love this change. It allows Shen to be even more team focused than before, and that's always where most of his power came from. His old Q and W were bland and didn't really help his team, they were more oriented towards his solo laning phase. By shifting most of his solo power to his new Q and passive, they freed up another skill for team-oriented play, which is fantastic.