This and other cleaning comments really have me going. I have not used a double action internal mix brush yet, but it seems to me that they would be harder to clean.
With and external, siphon fed, you just run some cleaner through the siphon tube and needle/tip and it's done. I wipe the outside of the tube and rarely have to disassemble the tip/nozzle, but that is easy to do and to set to soak in lacquer thinner.
An internal brush seems like paint can clog up inside the body. They have cutaway handles to pull back on the needles to blow out clogs as well as paint picker caps to expose the needle to pinch paint off from it. You might need to use interstitial teeth cleaners to clear out air porting holes. How is this all easier? I guess I'm thinking that cleaning the body could be a nightmare, if it's not thoroughly cleaned between uses.