Think of it in terms of efficiency and stability. Sending 1 or 192837198312 messages to one part of the body is fine, but having a AI system, cross reference itself, to the point where it would know how to react when different parts of the body send back errors etc..
It would need to be something akin to the human brain in complexity, being self aware and such. In which case we are going to boarderline terminator Skynet concept.
Giant robots just aint gonna work for a looooong time to come. We can only hope that some of the super advanced fantasy concepts in mecha and such are something possible in reality.>>9543662
So make it enough, you would have to have a person temporarily switch their own body over to that of the machine, effectively leaving all concious control of their body etc...
This is almost ridiculous as of current, but let's say for concept's sake technology will fix this dilemma.
Then you have to design the machine's body EXACTLY the same way as the human body.
Think of it in terms of current emulators that are being made for PCs for Gamecube etc..
Half of them don't run because the conversion from 256bit processing to 32bit OS systems is fucking insanely complex.
This will be a similar dilemma but on stellar levels.