For compartmentalization or for sandboxing I would go straight to Jails, solving some problems with current approach of simple & single network namespace Suite designers are having with browsers. Jails are very powerful yet easy to configure and maintain and quite light. Just to say, you can run jails inside a jail, you can have vnet, you can run pf and bhyve (the FreeBSD powerful type 2 hypervisor) inside a jail, full ZFS support and much more.
For the fine grained "capabilities", if the old MAC modules and trustedBSD are not enough (they offer something like 400 different, specific privileges), then I would say capsicum, which "hybridizes a true capability-system model with a Unix design and POSIX API" in contrast to Linux "capabilities" which don't. In reality they sound more like privileges oddly renamed as "capabilities" in one of the Linux twists of names and features (here I will save you from other Linux horrors because such horrors are caused by distributions jungle and not by the kernel itself). capsicum compares to Linux seccomp as well but it is definitely superior as you might see.
A more comprehensive and I would say pleasant overview of Unix sandboxing techniques was illustrated in 2017 in this (in my opinion) good looking article. https://freebsdfoundation.org/wp-content/uploads/2017/10/A-Comparison-of-Unix-Sandboxing-Techniques.pdf
In the above article you can find a couple of paragraphs emblematic to understand the choice by Suite designers and probably instrumental to the OP (your) question:
I guess that's all clear now. In other words: if Suite designers want to keep traffic splitting as simple as requiring a single network namespace (I guess (?) to avoid the serious problem caused by Mullvad's cgroups based solution), with or without true process isolation / boosted virtualization they anyway need CAP_SYS_ADMIN for cuckoo tool. Here you can understand the need for precautions too, crucially and obviously: forbidding root running, dropping setuid privileges, setting the privileges of the user running cuckoo itself.
All the best!
-- revsplus