We do lots of Carbon Cleaning, the walnut system we have is really good at cleaning inlet ports on Direct injection cars, where there is no fuel washing over the inlet to keep it clean, most DI engines suffer from carbon build up.
Our tooling fits over the inlet port, fires walnut shell into the port and then sucks it back out. You need to turn the engine over and ensure the valves are shut on the cylinder you are cleaning.
The idea of walnut shell is that if small pieces were to enter the engine it won't score the bore etc and will burn, you certainly wouldn't want to fire sand or glass beads into the inlet port with the engine in the car.

HHO we also do, we do see some MPG gains from it, but don't sell it as a carbon build up fix, as it only works for inner cylinder, more as a service to do every 10k miles to help prevent inner cylinder carbon build up. it won't cure carbon deposit issues once its built up.

Wet carbon cleans like terra clean I avoid at all costs, as I've had a few in for repair that have been done, and lumps of carbon have broken free and stuck behind valves, preventing the valve closing correctly causing piston to valve contact.

Water Meth is a great addition to DI petrol and diesel engines to keep carbon build up to a minimum, I run a kit on my 123D and see 25BHP gain from some meth.