When reviewing assignments, editors are on the lookout for three things: One, that images are sized and attributed appropriately (and that they fall under fair use), two, that the work is free of spelling, grammar and mechanical errors, and three, that the style guide specifications for each client were followed.
If an editor is requesting a revision, it is likely because of one of these three issues. An editor will spot-fix minor errors, but if the assignment is plagued with them, it will be sent back. If photos contain watermarks, are the wrong size/proportion, or lack proper attribution, the assignment will be sent back. If the assignment does not follow the client’s style guide, it will be sent back.
An editor may also occasionally send back an assignment if the direction or tone need work, but this is pretty rare.
Usually, if an editor is sending an article back it will be a quick fix -- but that depends on how well the writer followed directions in the first place!
Additionally, revisions are unpaid. This means it’s super important to check the style guide, do images the right way and write the best possible article you can the first time!