My experience of learning French vs. trying to navigate Paris was that no one in Paris actually speaks whatever "French" any of my classes had prepared me for. I very quickly discovered out dialects were more or less mutually unintelligible. Modern Parisian French more or less operates on mumbling and context clues. Even Parisians I've met have confirmed they don't always even understand each other.
I had a similar experience trying to learn Finnish. Spoken Finnish and written Finnish are completely different dialects, but the way it's taught in any official language course is basically "if you read written Finnish out loud."
So if you speak to Finns the way they teach you in school, Finns will probably understand you, but be thrown off because you'll sound like a weirdo. Anything they say back to you however, will sound like broken nonsense.
Because the thing is that Finnish grammar is infamously impossible, and so in common use, most Finns just ... don't use most of it. In fact, because of how Finnish grammar works as a language, quite a lot of it is unnecessary, and can be dropped or contracted without losing any actual useful information, so why bother with the effort? This is a language with separate prepositions and tenses for indicating an object depending on whether you're physically pointing at it or not. Nobody needs that.
The kinds of bureaucrats who design an official, state-funded language curriculum, however, want things to be "correct" ... so they don't even hardly acknowledge that spoken dialect even exists until nearly a year into the program, and if you try it yourself too early, the teachers will correct you relentlessly for saying things in the way you'll hear on every street corner.
In my first class, there was a lovely Bulgarian girl who'd moved here to be a hairdresser a couple of years back, and had already picked up a reasonable grasp of spoken dialect.
The teacher would constantly correct her every other word, and she got so despondent she eventually just dropped out after a few weeks. Just completely crushed her spirit, for the crime of ... speaking the language like an actual person. She ended up moving back to Bulgaria.
Incidentally, if you ever meet any Finns and are confused why they are so anxious about their English despite speaking it better than most Americans, it's because their English classes are run the same way.
