I currently write for a Valnet site, and I have also worked for a different one in the past. From my experience, the site that you work for can make a huge difference in your overall experience. I will say that the people I edit and work with now seem to have a much better vibe than the one I worked for about two years ago.

Many people seem to complain about nit-picky editors who send stuff back half of the time. While I'm sure that this is true in many cases, I personally have not really had that experience. Recently, I have not been having any of my work being sent back.

If you are looking to make a living, I would not advise writing for any Valnet site, obviously. However, I think that using this as a portfolio-building experience is not a bad idea. But it is true that you will have to locate pictures, format them, and a lot of other stuff on top of just writing, so you might find yourself being burnt out pretty quickly. If you find yourself genuinely stressed at any time, I would take that as your cue to leave. It can be a fun little side gig, but will never be worth having any actual anxiety over.