I started learning Polish with Duolingo but gave up when they changed the format a year or so ago. They were saying "it's about progress, not perfection" and took away the ability to go back and repeat lessons from earlier in the course. Like Oldnproud I am used to learning languages very intensively and to a high standard and can't give up until I understand the grammar completely, so a superficial knowledge of the language was not what I was aiming for. Also, as I round the last corner on my way to 70, it gets harder to retain things so I needed to keep going back.
I hadn't even realised that you could finish a course, but I suppose at some time there are no more lessons.
With Duolingo it also depends how popular the language is. The more common ones like Spanish have (or at least had ) all sorts of features, like stories and audio feedback, which weren't available in the Polish. Perhaps they are only for paying members, not sure about that.
When they disabled the discussion section, which was one feature the Polish did have when I started, that was the beginning of the end. People were asking serious questions and there were a few moderators and native Polish speakers who helped and explained things really well.
I didn't find the content as unrelated to real conversation as it seems to be nowadays, but that may be because I am used to transferring constructions for other uses when learning a language.
By that I mean: "My hovercraft is full of eels" doesn't make sense, but you can replace the words with others and use the construction "full of" to express something else, such as "The world is full of wonders". "Full of" does not come naturally to speakers of some other languages.