If you want to provide a redirected environment for a presumptively non-malicious process, fakeroot works fine. It's not a secure sandbox, though, because it's based on intercepting system calls with LD_PRELOAD, and a process that wishes to can avoid being intercepted in that manner, since it's just enforced by the shared-library loader. So processes that are statically linked, or that have direct syscalls compiled in, will bypass the LD_PRELOAD replacements. The seccomp mechanism, by contrast, is enforced by the kernel.
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If you want to provide a redirected environment for a presumptively non-malicious process, fakeroot works fine. It's not a secure sandbox, though, because it's based on intercepting system calls with LD_PRELOAD, and a process that wishes to can avoid being intercepted in that manner, since it's just enforced by the shared-library loader. So processes that are statically linked, or that have direct syscalls compiled in, will bypass the LD_PRELOAD replacements. The seccomp mechanism, by contrast, is enforced by the kernel.
the point is that mbox only uses seccomp with -s and most of it (ie the rest) is what fakeroot does already.