I remember being told by college professor to avoid drawing when making my book covers. At the time, I was only a sophomore still learning how to use Photoshop with a Wacom tablet. I know that I still have a lot to learn.
When I'm designing my book covers, I'd sketch the layout and then refer to a personalized color scheme to pick out some colors. I prefer to make my own illustrations so they don't end up looking too similar to others' works.
Just stick to only two or three of them so they don't look too busy for the eyes. I always like to add a few images to my covers since plain text just looks boring.
As for the fonts, only use two different ones but make sure they're eligible. If you're a romance author, you can use a cursive font for the title and apply the Times New Roman font for the small captions. You can use any preexisting image related to sweet things found on Pinterest as long as you credit the original source. Just avoid using racy or inappropriate images as possible.
I had come across book covers on Wattpad with confusing layouts and ineligible text.
Second, there is barely any contrast (e.g. light background plus muted font colors).
Third, some authors don't even credit the original source of the images used for the cover.
Fourth, too much clutter that it became a lot more of a collage. This includes too many cliparts, glitter, text, etc.
Lastly, this may sound prudish but I am bothered by the use of racy and inappropriate book covers (not to mention the authors are also young).
Ah, let's not forget about the background! It's best to stick to flat colors. Gradients and prints are fine as long as they aren't too distracting.
To sum it all up, make your book covers look engaging and unique. They are what draws your potential readers into your works.