Yes, I consider the human factor. The argument is largely manual vs automatic. Do you want a system to handle things for you, or do you want to handle everything yourself? The thing is, with experience and practice the error margin of input mistakes becomes incredibly small. A person might still make mistakes in their judgement and foresight, timing and mechanics, but action input mistakes on high levels of play are very rare. That's because humans have in-build automation systems in them: muscle memory and scripting (psychology term) of repeated actions. Instead of transfering that muscle memory to a macro, you can keep it in your body, while retaining the ability to be flexible with your input. You brain is way more flexible than the macro system in game.
You also have to consider that the macro system is inherently flawed vs manual input. So, when both sides play perfectly, manual input will always pull ahead. You'll have to consider the chances of input mistakes of both sides, but manual side always has to do more mistakes for the macro side to catch up. You also have to consider the nature of the mistakes both are subjected to. Not all mistakes are related to one's input method. Even if you argue that handling a broader input spectrum would take away from your mental capacity to avoid all kinds of mistakes, you have to remember that muscle memory and practice can take away essentially all overhead related to action input, leaving you with the same capacity to process the rest of the fight.
I'm arguing for the end state of practice. You will end up with essentially the same error margin on both input systems after practising a lot, the difference is that one input system is more versatile and effective than the other one. KISS is a common saying in a lot of fields, not just the army. It's most suitable for situations where people need to pick up new skills very quickly and react under unexpected stressful scenarios. FFXIV gameplay is neither of those. All encouters in the game are very scripted (except PotD), you can practice as much as you want, and do it safely from your home. It's true that you can reach your "complete state" more quickly with a simple macro system, but with ample time and given the fact that you can practice manual input in more scenarios (all classes), you will reach the same state of performance inevitably without it, possibly faster if you also play other classes besides MNK.
The question is, when you're at your best after practising, do you want to be using a flawed system (even if it was only 2%) or the most efficient one? Both will eventually end up with similar error margin for input mistakes.