By morally, he might mean it creates a moral hazard. I know that when I was poor, I worked only the minimum to support myself. If I had UBI that covered those costs, I certainly wouldn't have worked, so there'd be less productivity in the economy.
Well, exactly that problem already exists qualitatively with current tax and welfare systems.
Whether UBI would make the problem quantitatively worse depends on the exact design of the UBI system you have in mind and the current system you want to compare it with.
