SMRs are being touted not because they are particularly promising, but because they are the last option nuclear supporters have. Traditional reactors have run out of chances, so now it's either the Hail Mary Reactors or give up.
Are there any known problems or difficulties with them? I guess the materials science aspect is extremely difficult as it is already a huge problem in conventional designs...
The big problem is diseconomies of scale in construction and operation. They require more material per MW of power, and more staff too. The story is that economies of scale of manufacture, and changes to regulations about operation, can counter this, but that remains to be demonstrated.
