Really hard. My maternal grandfather's family in Finland are Swedish Finns (ethnic Swedes living in Finland since many generations, the remains of the Swedish Kingdom's occupation of Finland centuries ago). Their first language is Swedish, however they are a tiny minority overall (about 10% of population at most). So they need to learn Finnish for school, for work, for most everything outside their Swedish-Finn families and communities. They start in 3rd grade in school formally, if not earlier informally. They will all tell you that even growing up learning and using Finnish as a second language from an early age - it is a difficult language.

Though for Finnish kids, as a first language, probably no harder than any other first language!