This is debatable. It's true that V8 is a much larger attack surface than Firecracker, therefore likely to have more security bugs than Firecracker itself. However, Firecracker runs attacker-provided native code directly on hardware, which means that hardware itself becomes an attack surface, one that is quite wide, not fully documented, and very hard to patch if problems arise. It's much easier to work around hardware bugs when you're working from JS / Wasm and can control the code generation.

Ultimately I don't think you can really say one or the other model is more or less secure.

(Disclosure: I'm the tech lead for Cloudflare Workers so I am obviously biased here.)