The Guardian ran a few alarmist articles on them going back a while which might have sparked some of the outrage, including this opinion piece [1]. It turns out these "dark satanic mills" are often: hot, without windows and cold in winter.

I don't advocate for bad working conditions, but it must be hard for anyone who has ever worked in a kitchen to keep a straight face when reading that. Even many Michelin starred kitchens are horrible environments.

[1] https://www.theguardian.com/commentisfree/2018/oct/09/dark-k...

Edit: This isn't meant to be whataboutism, just pointing out that a "ghost kitchen" is only really different from a normal kitchen in its business structure. By and large, catering is not, and has never been, a glamorous industry. If you want to stop kitchens being difficult places to work, that's a worthwhile cause, but much, much bigger than Deliveroo.