1.- Videos more than 25 min.
Unfortunately, I’ m not able to reproduce your problem.
As a test, I just imported this YT video from “Deutsche Welle” (subtitles in US English, length: 42:25 min) via the LingQ Google Chrome extension:
It worked like a charm. But, the video needs transcripts / subtitles and its length should be less than 54 minutes.
What doesn’t seem to work is importing the audio from Netflix. Only the subtitles are imported here. But this fits my learning style, i.e. 1) watching an episode without subtitles or with English subtitles 2) working on the transcript in LingQ 3) watching the episode several times with / without subtitles.
BTW, Netflix and LingQ is an “excellent” combo for language learning, because Netflix offers thousands of series / movies for many languages!
2.- Books.
I usually find all the books on IT, social sciences, history, politics, etc. I’m interested in and can convert them into the right format…
As others have already suggested https://epubor.com/ is your conversion friend in this context.
3.- When for some reason the importing feature does not work (in my case 5-10% and this is significant in my view.)
Do you get any error messages?
But, I wouldn’t say that an error rate of 5-10 per cent (or: in my case: 1-5 per cent) when importing text / AV material is highly problematic. This means, for example, that you can import 90-95 (in my case: 95-99) texts from 100.
And I’d say you can reduce your import error rate even further by using conversion tools à la Calibre, epubor, etc. correctly.
But there is also a lot of people that is not so good when finding the content they need, and I think it would be helpful for them to find here the material well filtered.
This may be the case. But from my teaching experience both in languages and in math (> 10 years, about 10k hours), too many learners are too passive. That is: They don’t take much responsibility for their own learning process, they tend to give up almost immediately when things get unpleasant, etc.
Unfortunately, an extreme form of this passivity can be seen in math and programming when learners (esp. teenagers) just want to imitate the solutions presented by others without thinking for themselves, without investing the time and effort to find solutions for themselves, etc.
In short, they don’t want to struggle and want to avoid pain/discomfort at almost any cost. But the paradox is: what protects us in the short term (avoiding discomfort) undermines us in the long term, because the pain avoidance mechanism can “ruin” our life in general and our path-of-least-resistance learning in particular…
Well, it’s a pet peeve of mine and I’m in the middle of writing a book on the subject. So, sorry for the rambling! 
Now I´ve realized that have to start reading and watching 50%+ of my learning content out of the tool.
Well, I started as a LingQ power user only a few weeks before you (in July 2020). And I’ve imported between 200-500 videos, blog posts, articles, audio files, e-books, etc. in five different languages so far.
In this period, I’ve gotten an import error maybe 10 times. Did this affect my language learning? No, not at all because I usually find a workaround…
But, I agree: (at least at the levels A1-B1) LingQ doesn’t work equally well for all languages.
In my case, it works like a charm for Indo-European languages, but not so well for Asian languages like Japanese.
So I probably spend 75 percent of my learning time on Japanese outside of LingQ because I focus more on the oral dimension. Later, I will focus more on reading with LingQ.
However, as a Spanish native speaker learning German you shouldn’t face the same problem with reading / writing!
The other part of the problem is that execution does not match the expectations Lingq sells.
Well, I’d say they’re a small team that is trying to do their best. But, from my own experience in software engineering I can assure you that things that seem obvious or easy from a user perspective are sometimes really tricky “under the (software) hood”
However, you make yourself unhappy when you consider LingQ the “only” language learning tool that you need.
If you adopt a “hub” perspective (see my reply to @ericb100) where you combine a flexible audio reader with a “zoo” of tools (in my case: Netflix, Anki / Memrise / Mosa Lingua, Deepl, Busuu, JPod101, etc.) you’ll be much happier because you’re more flexible regarding tools and methods.
Or to put it in Tolkien’s “logic”: Sauron’s mistake was that he thought “one” ring could rule them all. The lack of diversification and disregard for the “lesser” creatures therefore provoked his downfall.
As language learners, we shouldn’t repeat Sauron’s mistake.
In short: “LingQ < > master ring”
Have a great evening (if you’re in Europe / Spain)
Peter