Everything has downsides, and it has to be compared to the alternatives.
In Finland's case, the realistic alternatives are burning coal or burning Russian gas. (If the Finns dedicated a substantial chunk of their forests to this one generator, they could maybe use biomass.)
Coal kills two orders of magnitude more people per GWh than nuclear--and it does that when operating nominally, not when malfunctioning--and it produces three or four orders of magnitude more waste and more environmental harm from mining.
Russian gas has geopolitical/national security problems.
Biomass is a roundabout way of burning diesel fuel and gas, while degrading and eroding forest soils and polluting watersheds.
The number of new meltdowns per decade rounds to zero, to five significant figures.