This whole Finnish nationalism debate is a bit of a sidetrack, actually. Scandinavian unification depend on Denmark, Norway and Sweden. Whether or not Finland goes with them is not a core issue, it's a bonus at best. The Finns were never influential enough within Sweden to block Swedish policy, so the worst they could do, is make the prospect of incorporating Sweden unattractive by making control of Finland more costly. In our timeline, if that happened at all, it was a very minor factor in the calculations of the governments of the 3 main states.
As we're talking mostly about the 19th century, the issue is the relation between nations and states. In earlier centuries nationalism played a much smaller role, so if the question were about the potential success of the Kalmar Union a very different logic would apply. But in the 19th century European elites thought that nations should coincide with states and if they didn't, they acted to make it so. Some of the time this meant supporting a separatist or irredentist movement. Most of the time it meant educating subject to become properly indoctrinated citizens. All European states at that time tried to reduce dialect usage and promote a standard language. They all introduced a national curriculum for elementary schools along with mandatory education up to age 12. Most of them introduced military conscription, putting conscripts in regionally mixed regiments. In this manner the nation made the state. There are some exceptions, and certainly efforts at nation-building weren't equally successful (or equally determined), but on the whole we're talking about the era of state-led nationalism.
From this perspective the unfortunate fact for Scandinavian union is that the area already had 3 states with relatively well-established borders and local traditions within them. Naturally these states worked to instill nation sentiment within their territories. Only Norway was not independent but it was governed as a separate dominion under its own laws. If Sweden had been a more forceful occupier, it could have tried to assimilate Norwegians, according them the status of speakers of really weird dialects with some strange practices due to their remote mountain valley customs (i.e. Norwegians = Swedish hillbillies). But Sweden didn't really try. probably because it wasn't strong enough to expend police and military power there while maintaining a strong defensive posture elsewhere. So they allowed a separate administration that at least didn't promote Norwegian nationalism but also didn't put much in its way.