I'm the original poster that said v8 isolates aren't a great sandbox and I would state that I didn't say they were impossible to use as a sandbox. I know you all at cloudflare are doing a ton on top of v8 isolates to make them secure.

I think as a general rule you'd just want multiple uncorrelated layers of isolation. If you aren't willing to spend what cf does on securing v8 isolates I think process isolation + seccomp + v8 isolates might be enough, otherwise all of our browsers would be ticking time bombs.