I have found this same problem, Items need a better explanation before purchase, I have downloaded a few that have a jpeg in an Ai file, there should be a way to comment and give feedback on usability of images. Or at least flag up content so other users know what to avoid. there is just no communal aspect to Adobe stock. even a preview of layers would do. Anything like this.
Branching into a new discussion
I understand the issues that you've described as above but when you open an image, the file type is already mentioned below where you can check it and then download it.
Also, if there are any quality issues with an files you can always report it and we can fix the issue for you.
Let me know if you have any concerns with any specific image so that we can check it for you.