It's true that V8 isolates are more risky than micro VMs*. However, it's also true that micro VMs are more risky than giving each tenant their own machine.

At some point you have to decide where along the spectrum you want to put the boundary of "acceptable" for your workload.

Some people argue that isolates are below the necessary threshold and micro VMs are above it. But this isn't really based on any rigorous mathematical analysis, it's mostly vibes. It used to be that people said VMs weren't secure enough for critical workloads, but few people say that these days.

I would argue that we (Cloudflare) have demonstrated that the isolate model can work fine if done carefully: we've been doing it this way for nearly a decade with no breaches.

* Not "strictly riskier", though. There are some risks micro VMs have that V8 isolates do not. Micro VMs that allow tenants to run arbitrary x86 code place a huge amount of trust in the hardware to be exactly correct; a trusted JIT makes it much easier to work around hardware bugs when they are found.