I have been on Zenfolio for one year before I started my Symbio site in 1993). I didn't notice any SEO at all. Traffic on my Zenfolio site has been about 1/20 of what I get with Symbio. No sales at all (compared to very few on my Symbio site). There was one serious interest by e-mail, though, which if it had led to a deal, would have been a killing. Didn't pan out.
Customizing my site, uploading, categorizing my port, their support and all was a 6-7 out of 10 experience. Not too bad for the price (at that time, at least).
However, I am glad I've leaped into self-hosting. It has been a steep learning curve and was time consuming but even if Symbiostock finally vanishes, I would never go back to solutions like Zenfolio or Photoshelter.
The next few months/ a year(?) will show which direction the new symbio/GRFX will take. Also, there's a chance that Woocommerce itself will offer a solution for stock. Who knows? - If my hosting plan was to expire soon I'd extend for at least another year.