DTrace works exactly like mdb, it requires knowledge about V8 internals.

The problem is haphazardly upgrading the V8 version without thinking about the consequences. This is not how you do release engineering, especially since io.js is advertised as stable.

Stable doesn't mean "it works for me", it means you provide some formal guarantees to downstream consumers about the stability of exposed interfaces.

I'm very well aware that some (most?) people don't care about things like good introspection support and stability, but it's pretty pathetic to see factual information being downvoted because people don't agree with the message.