I have a new client who is running on (paid-for) Wix and my auditing software can't audit the site.
Looking at the source i think it's probably because it's mostly scripts, no outbound links, basically nothing indexable.
I remember Google saying they would stop crawling Wix sites back in 2015/16, is this the case ?
Here's what I dug up.
https://productforums.google.com/forum/?hl=de#!topic/webmasters/YFvytjy2gJg;context-place=forum/webmasters
http://webenso.com/is-wix-seo-friendly/
I've not looked closely at them in a while, but I wouldn't touch it. It will hamstring you in one way or another if not several. I'd get them on a proper platform before starting work.
Same goes for godaddy site builder, weebly, and all these other drag n drop/gui builders. Not SEO friendly.
I've had a few small clients over the last 18 months who were on WIX - always the first part of the strategy is to get them off WIX