Good question 🤔

Here is ur answer

Like normal Devil Fruits, SMILEs render those who consume them unable to swim. It has been noted that, unlike real Devil Fruits, SMILEFruits actually taste good.

So those that can never cry or be angry... can never swim.

That is a curse is truly worst than death.

Yeah you really get screwed with the failed smiles, can’t swim, no other emotion and no power.

I wonder what would happen if they ate a real devil fruit

In the worst case scenario... the same fate of those who eat two devil fruits. (Unless you're Blackbeard.)