Biggest issue with VN/eroge adaptations is that there generally needs to be a generally linear storyline, major event that happens in all routes, or a true end to qualify to be good, and frankly, a lot of them don't have that. In addition, the producers need a good idea of their target end instead of just winging it. I've seen too many instances of 6 episode romcom, 5 episode character story rush, last episode or two drama with cover girl end. Best example I can think of how a difference in planning and execution can improve an eroge adaptation would probably be between Toei's Kanon (2002) and KyoAni's Kanon (2006).
As general flaws for each adaptation type goes:
Manga adaptations have a bad tendency of getting picked up while running, so filler and cutoff endings occur a lot. In other cases, longer series will be undercut to fit a 13/26 episode season.
Light novels tend to lose a lot of detail going to animation, and are also prone to the same issues as with manga, though usually there's enough between books to be able to make it work for a short season due to them being more episodic.
Visual Novels and Eroge have an issue with being similar to each other, and after having seen the better series out of the bunch, most other series can feel like more of the same with different tropes applied. In addition, either story and/or character development will usually take a hit since most adaptations are slotted to 13 episodes.
Originals are very much hit or miss and can change dynamically based on ratings.
And in general, most endings suck. Not many series have a good solid conclusion.
I enjoy them all the same, though usually manga will come first since that's what I read the most, so familiar names will pop up. Next would be visual novels and eroge since I'm generally familiar with which companies make decent games. Light novels I generally don't keep up with much, but if there's one that I've been following, I'd probably like it, and originals tend to fluctuate a lot cuz I'd actually have to do research to figure out if I might like it based on staff, and usually I'm too lazy to do so.