It varies quite a lot by airline, airport, and individual agent, and in my experience intra-European flights are the most rigid about it. I've only flown Finnair once, and I don't remember them taking any special note of my rather heavy rollaboard, though I'm quite sure it was over 8 kg. (This was a transatlantic flight, though, not within Europe.)
If you give it a try, it would be wise to have a Plan B in case you're forced to check something. Also, they may weigh your carryon bag but not your backpack, so you might try to offload some of the weight to the latter to keep the bag weight down. (I think the only airline that ever weighed my backpack was the completely appalling Clickair, which along with Alitalia constituted my "don't you ever ever EVER book me on this airline" list.)
Oh, and an unsolicited pro tip: Finnair's galleys normally stock cloudberry liqueur. Good, good stuff.