Hi Bill, I create illustrations for stock sites (Shutterstock, iStock, Fotolia and many others) entirely in Affinity Designer (since March 2017). I don't use Adobe Illustrator or any other vector editors no more. I can not say about gradients and transparency because I use only flat colors - and all works fine. Now a little about my workflow:
1) You can see my export options on screenshot attached.
2) Sometimes Shutterstock by mistake doesn't accept my illustration saying "EPS Format -- EPS file must be compatible with Illustrator version 8 or 10", so I upload it without changing anything once or at most twice again (without choosing "Previously submitted content" from "Notes for Reviewer" dropdown) - and they accept it.
3) Importantly, I use vector canvas 400 px on the largest side and 400 px or fewer on the smallest side, otherwise they say at Shutterstock that the canvas is too big or too small (see about this requirement in this article).
4) EPS created in Affinity Designer is small compared to the one created in Adobe Illustrator, so some microstock sites check it's size and give the error message. So, to get illustration accepted on all sites, I add some abstract text into the EPS file to grow it's filesize. I attached here the file with my "text for size". I open EPS with text editor (I use Notepad++).
5) I use Irfan View instead of Adobe Bridge to add metadata for JPG files.
So, Affinity Designer is the best choice for me. It's so cheap compared to Adobe Illustrator. Moreover, it's faster, more handy and more customizable. Additionally, Affinity Designer is great for isometric illustrations. You can look at my portfolio at Shutterstock.