Hi, everyone! @Braden-Hallett asked me the same question about 2 weeks ago but I haven't got the chance to reply back. Now that I'm here, I'm going to share my experience with Advocate Art. Before I get started, I'd like to say that everything I'll be sharing here is based on my own experience. Other artists might have a different time with Advocate. This is what just I've experienced so far.
Regarding how I've been treated, I'd say it's been really good. My agent has been really accommodating to my questions and queries. She's been really patient with me. They regularly brief us on the current trends of the industry and encourage us to make new pieces based on their list. My agent having an illustration background, gladly provides valuable critique on my work as well.
Now as for @Kevin-Longueil 's question about the top artists getting the first pick and the lower artists getting the scraps, I'd say it's really true. It might sound unfair at first but looking at it from a business stand point, it's actually logical as @NessIllustration pointed out. Top artists have been in the industry longer. They most likely have a decent following that'll help boost sales. They also have a proven record that'll make them less of a liability for clients compared to newbies. it's a bit frustrating when you're just starting out. Luckily though, it's not permanent. Once you've proven yourself, things do get better or so I've heard. 😅 well, I hope it does.
As for me, I'm still on the proving myself stage. I signed with Advocate around September 2019. I received a few queries starting a few weeks after that but I didn't land any projects. It's not really the agency's fault. There could be a variety of reasons why that is so ranging from my style just not clicking with the client to another illustrator just being better than me.
Anyhow, It was around January when I finally got my first illustration project. I was for a 16 page textbook for an educational publisher. I had a few hiccups on this project but I'll spill the tea on that one some other time. I got a second project last February from Harper Collins for another 16-page text book. This one was a better experience than the first. Needing only 8 illustrations plus cover, textbooks are low risk projects great for new illustrators like me but the quick turnarounds are what gets you. After the revisions, we completed both projects around last month. I received another book inquiry after that but I'm still waiting for news. There was no way I would've found these jobs on my own so I'm still thankful.
I'm currently keeping myself busy by finishing up a project from a self-publishing author that I picked up last year. We're on the final stages. I'm also trying to think of ways to get myself out my low stats. I think I'll try @NessIllustration 's advice of allowing my agent to manage the projects I found for myself. A publisher I worked with a few years back contacted me. I think I'll refer them to my agent.
Anyway, to wrap it up. Advocate treated me nicely, they have a lot of established artists so be prepared to work with small projects at first, but things can improve if you show them you're reliable. I hope this helps.