One of the biggest things that can cause trouble when creating an anime adaptation is the change in mediums from one thing into anime. Why does that cause trouble? Anime is a very timed and efficient medium. Each anime episode is approximately 21 minutes without openings and endings which means that 21 minutes of content needs to be put into each episode. When a series is one cour or 12 to 13 episodes, there is only so much content that a studio can put into those episodes. Approximately 252 to 273 minutes a season. That means more and more time as additional cours and seasons are thrown into the mix to adapt a certain work. It sounds like a complicated thing.
Manga Adaptations
From the medium change, it feels like manga would be the easiest thing to adapt in anime. Possibly why there is so many manga adaptations out there. Manga is a comic after all so there is some writing and visuals connected together in a way that one can see it be an anime already. Especially since a lot of anime use manga panels at story boards. The hard work of coloring it, animation it, adding sounds, and voice acting follows after that fact. I mean, that is the easiest thing to do and yet that takes hours and hours of work to accomplish.
What if a creator wanted to add something more to do? What about choosing what parts of the source material to animate on a time budget. Manga can have all the time it needs to tell its story, but anime is watched by people on television or by streaming weekly. Are there parts of a story that do not belong in an anime that need to be cut out without missing part of the story? What are those parts? What is the maximum amount of context that can still create some drama. I imagine that a lot of shonen battle adaptations have some more time to tell their stories but there are still cuts that need to be made. Fun times.
Light Novel Adaptations
Making a story out of written word and putting it into animation, so it’s fun. I feel like light novels have a leg up on novels that were created into movies because light novels have some importance scenes and characters drawn out so you know that you can see something out there. Still, there is the matter of choosing what to adapt and how to adapt it. What is the studios interpretation of important moments and is that accurate to what the author had in their mind? In my very unresearched mind, it feels like there is more give and take in finding a golden mixture to creating an entire story that has unlimited time into animation.
And honestly, a lot of the adaptations that I’ve seen coming from light novels are writer heavy. I don’t just mean with character dialogue, but explanations of the stories brought in themselves. For example, how is a fantasy series going to tell you about the world its presented. Will it give the watcher an extensive load down that they are going to quickly forget or will it tell you more about the world over time? Hard things to consider here. Honestly, this fact is why most of them aren’t for me. I know that I am a Tomino fan and his dialogue is expository in nature a lot of the time, but I don’t think that a lot of anime have captured that sort of dialogue with as much charm or humanity. That probably is a bias though and I should probably give them a bit slacker since the studio is creating the anime from almost scratch. Laziness doesn’t exist in the anime industry.
Video Game Adaptations
I am not going to lie but studios that turn a game that lasts for hours and hours into a one or two cour anime series have the hardest task possible. I feel like that is so much of them fail. Seriously, how do you tell hours of story in small bits of time? How do you capture the player’s experience for those who haven’t played the game? The answer is that you can’t. Well, at least not easily. I also think that limiting a story in such a way through this time spring can hurt an anime as well. I can’t help but feel sorry for people tasked into creating an anime out of a video game as being some of the hardest working people ever because they are given an impossible task. I mean, otherwise the characters, stories, and visuals are all there ready to play.
The main thing I see is choosing what audience to create an anime for. Do you aim for the people who played the game and animate the best action moments with bare minimum context or do you create an adaptation that everyone can enjoy with as little amount of time as possible? There are some cases like Rage of Bahamut: Genesis and Virgin Soul which create their own stories out of what they are given from a Gatcha Game, but not all anime adaptations have that chance to just have some fun. It really does feel like a lose lose game here. Poor people tasked with accomplishing all of these series. Damn.
Conclusion
Creating anything difficult is hard. I mean, original anime are hard because everything is started from scratch. Character designs, story, themes, story, and other things. Anything that involves a bunch of talent working hard together slaving away for hours to create something special deserves a lot of credit. I don’t think any difficulty changes when an anime turns into an adaptation just how much creative is approached to it. The level of effort behind each is still decided by the studio behind it and the resources and budget they have behind it all. And no, these aren’t thoughts that are well researched at all but they do come from how difficult and obvious it all is.
Mechanical Anime Reviews 
So I had to read this twice and come back, because I knew I had thoughts but originally lost what it was that I wanted to say.
I agree with you that adapting literally anything into an anime is hard. There’s so many factors and elements that you’ve already mentioned that goes into it. I think one thing that comes to mind for me, and you mentioned a few times indirectly is actually adapting the story from one media to another. Like you mentioned with fantasy stories, the 1-3 pages of info-dump and context aren’t a big deal in a light novel. Having 10 minutes of a 23 minute anime episode dedicated to this, rarely works out.
For me, I’m not so interested in a beat for beat adaptation most of the time, but seeing how the same story plays out in a different medium. Trying to cram every single detail, while it makes the purists happy, just isn’t realistic in most regards. Too many shojo manga to live action adaptations have shown as much lol. But seeing what a film director, verses anime director decides is worth including or making a point to focus on is always interesting. There’s still the key factors and original story holding the two versions together, but how they play out is interesting.
The best example I can think of being Kiki’s Delivery Service. I’ve seen both the Studio Ghibi film (forever biased), and the live action movie. Still working on tracking down the original novel so I can’t quite make a full comparison. Despite the live action being incredibly different, a lot of CGI and the events were trimmed down I think, the overall ‘feeling’ of the title remained the same. For me at least, it still gave me that warm fuzzy feeling that the animated production gave me, as it followed the same characters and story just a different interpretation.
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I’m in that same camp of wanting to see anime directors put their own special spin on stories from all sorts of source material. Straight adaptations can be ok, but the flair a good anime director could give their source material really can’t be denied.
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