Earlier quoted context omitted.
I'm pretty sure, that the BSD family is pretty mature and secure. Linux is just good enough for most people.
>is pretty mature and secure They are still missing something like capability based security like iOS and Android have where apps have to be granted access to use things like files or the camera. It may have been considered secure a couple decades ago, but they have fallen behind the competiton.
I wouldn't consider any of these systems "secure", though, as a practical matter. In terms of preventing a breakout, I'd trust an application on OpenBSD with strict pledge and unveil limits, or a Linux process in a classic seccomp sandbox (i.e. only read, write, and exit syscalls), more than any of those other systems. Maybe Capsicum, too, but I'm not familiar enough with the implementation to know how well it limits kernel code surface area. But any application that can poke at (directly or indirectly) complicated hardware, like the GPU, is highly problematic unless there are proofs of correctness for any series of inputs that can be sent by the process (which I don't think is the case).