Hello,
Certain things to point out, since your points are essentially repetitions of a couple of concepts:
- Charts not being on time; some of the dan courses were made in a period where timing is really difficult for me, since I used a program that focused on waveform rather than sound for timing. I understand some of the concerns, though many players who play through these courses will rarely notice this because of how they play mostly visually. Incompetency on my part, which is also why I'm not planning on making any in the future.
- Certain charts being terrible, or imaginative (the proper term is "usage of ghost notes"). Most of the charts you mentioned (barring Stinger) fall under dump charting; this is essentially an expansion of conventional charting where charts do not adhere to your standard "every note must be justified by one particular sound" and "chords cannot be this heavy because charts of other music with similar intensity only have chords of this density". It's more about conveying the essence of the music (for playability purposes, usually to make charts that are harder than the music is given) while conserving some level of musical relevancy. You shouldn't be looking at charts like Can't Take My Eyes Off You, Kaguyahime and Go For It with the same lens as you would for your standard ranked chart.
This is not to say that some of the charts here are not very good of course. I don't like playing through a good portion of these charts myself, but there are not so much there for playability (though it's certainly a "best of the worst" situation), but moreso because they fit a certain niche in the course. Additionally the reception for these charts from players are typically positive if the charts are in isolation, so to the general community, they find a good portion of the charts enjoyable to play through.
You can't really say the same for most of the charts here if you're looking at it from a charting perspective, which you clearly are. I understand the concerns if you're looking at it from a charting perspective and I agree with some of them, but again, you're not the target audience here. You can easily plow through 85-90% of the charts here and criticise it for having layering inconsistencies and poor pattern selection (and I could as well), but what's the point in being pointlessly analytical if players (i.e. the target audience) won't even notice the nuances and consistencies that charters value so much in the first place?
- Charts being difficulty spikes have been acknowledged elsewhere and I've admitted to that many times. Other dan courses have been made to try to amend for that, but they are also having problems of their own. Again, incompetency on my part, and it's also why I don't like dan courses as a concept now. It's not easy to find charts that are of suitable difficulty (relative to other charts), especially if the person who is selecting the charts has a skill level far above the skill level needed for the dan courses themselves.
The main reason why dan courses got so big in the first place is because it's an easy way to assess skill. Many players have disputed the PP system as a poor system due to a small selection of ranked charts (consequentially being "who is the best at a certain set of charts", though dan courses aren't too much better in that regard) and lackluster difficulty system. This works probably as well as it can be since players consider this to be a common language as to how good a player is. There are some errors in the dan courses of course (sync, spikes), I'd absolutely be the first person to say that, but it doesn't stop people from overlooking these imperfections because these problems aren't all that significant to them. There are exceptions to the rule, you and several others included, but it's still a minority.
I don't expect you to change your mind about these dan courses not being a useful assessor of skill. I think dan courses should be something people should take lightly because of how it's essentially "how well you can do these 4 charts of varying skillsets", but again, it's the closest osu!mania has to an actual assessor of skill. That is the only reason why it has gained as much traction as it did. I think calling it a bad way to assess skill is incorrect to some degree, but most of your criticisms about chart quality (which is different from enjoyability), sync and difficulty spikes are sound, and I agree with your points there. It's just that again, you're fixating on the wrong aspects of what makes dan courses so appealing to players in the first place.
Hope that gives you some explanations on why dan courses are so big in the first place, despite all of these flaws that you've pointed out.
in the 8dan v1 and 95.2acc in 9dan v1 
