Finns were allied with Nazis. Also when they attacked USSR in 1941 they were stopped by Soviets just like they stopped Soviets a year before that.
Soviets withstood joint Finn-Nazi offensive and won the war...
After a 2 1/2 year stalemate (Winter 1941-Spring 1944), the Soviets went on the offensive and recaptured much of their lost territory (which was actually the Finland had lost to them a few years earlier) before being halted by a pair of decisive Finnish victories that allowed them to sue for peace on somewhat favorable terms.
Hitler refused to accept their withdraw from the war, so the Finns spent the last eight months of the war forcing the German 20th Mountain Army out of extreme northern Finland (Lapland).
I'm not saying that the Finns were completely blameless for their actions in WW2, but they certainly had legitimate grievances with the Soviet Union. Since the Finns did effectively switch sides (and the American and British governments were sympathetic to their grievances with the Soviets), they were given little more than a slap on the wrist by the terms of the Moscow Armistice. They had to formally accept the loss of the territory they'd ceded to the Soviets in 1940, pay $300,000,000 in reparations to the Soviets (later reduced to $226,500,000), legalize the Communist Party of Finland, ban all fascist political parties, and accept set limits on their military.
Unlike the Serbs.