Native Finnish speaker here...
To answer your question, no, I don't perceive Finnish as complex
But I guess no native speaker of any language would find their own mother tongue complex, or even be able to perceive its complexity... Although I do realise that the grammatical structures and the syntax are different from Indo-European languages, which are the point of reference for most non-native speakers. But I think that's cool, it gives perspective to learning languages, knowing that things can be expressed in many ways.
Native speakers obviously usually don't make mistakes with cases. As regards noun inflection, there are some old (often agricultural) words that are not much in use anymore, where mistakes occur nowadays (e.g. ori:oriin, ruis:rukiin). As for correct case usage, only in some very complex sentence structures, such as when using non-finite clauses ('lauseenvastike'), there may be some confusion regarding which case to use. But these rarely come up in spoken language. Kids also learn the inflections and correct case usage along with everything else, so I think already the youngest kids who are able to form any kind of coherent sentences would be able to inflect nouns and use cases more or less correctly.
The short/long distinction in vowels and consonants is clear and unambiguous to a native speaker also in spoken language.