Privacy Guides’ main priority is recommending products, services, and practices which better protect people’s privacy. It seems they try to have some balance with usability and security as well. The main selling point of Zen over Firefox seems to be that they offer a different UI/UX which isn’t a privacy benefit over Firefox.
Just because something isn’t recommended by Privacy Guides doesn’t necessarily make it a bad option, it just isn’t currently an ideal option for what PG prioritizes. If you do stray from their recommendations, do so understanding that researching the privacy-related risks, pros, and cons become your personal responsibility if that’s something you care about.
This might be a good reason for some to avoid Zen Browser but it’s really up to your values and threat model. Another thing I’d look into is how quick Zen Browser manages to push Firefox updates in their browser. It’s not uncommon for downstream projects to have large delays in receiving security updates which could be a serious security concern.
Assuming you’re referring to the recent Mozilla controversy, I haven’t seen the Privacy Guides team comment on whether they think this is a privacy issue or not. While many are jumping on the anti-Mozilla bandwagon, it’s also worth hearing some differing perspectives:
Nicco Loves Linux offers some differing opinions and insights in his “Everyone’s Wrong about the Mozilla Situation” and “TheLinuxExperiment ditches Firefox for all the wrong reasons” videos. Mozilla also offered some clarification on what happened and why. I’m sure this wasn’t enough to satisfy everyone so I’m waiting to see what more Mozilla has to say about the situation in a potential upcoming interview.