We know there are grave short term effects from radiation, but the effects 20-30 years out have always been a guessing game.
It depends on how much radiation one gets. Those first responders at Chernobyl got massive doses that were fatal pretty fast. Overall I personally believe the evidence supports radiation homesis at lower levels.
We actually have some data now. Though not statistically significant, the populations around the Japanese nuclear bombs that survived the initial destruction, have had slightly fewer cancer deaths than the rest of Japan.