And the issue in using that tuning tool is: The line between "easy" and "hard" is very thin.
Typical human reaction times to stimuli lie in the deci-second area, so a measly half a second more or less can make a huge difference in the difficulty of a mechanic. Add latency to the equation and a mechanic that is meant to be "difficult" to avoid might well be impossible to avoid for some, while designing mechanics with higher latency in mind can make them too easy for people with less.
This makes it a fairly bad tool for difficulty tuning, especially in an online game. You're prone to get it wrong one way or the other.
That in turn is why heavily scripted fights are usually the go-to choice: By having set things happen at set times, you can be proactive about it and cut down on reaction times - If you're already standing in the middle when Hashmal jumps, you'll probably have ample time to look around and get to the safe zone, because the way is short. If you don't, you might be standing too deep in the wrong side and simply cannot make it even if you react instantly. And I wager the main reason why people fail that mechanic is because they simply don't know the script yet and thus don't pre-position themselves, because the timing is very lenient if you do, the timing on most mechanics is from my experience.
This circumvents the issues that come with reaction time tuning somewhat, because learning the script takes most of the reaction difficulty out of the mechanic and this shifts a lot of difficulty to learning the script by heart instead. When I tried out raiding, it was therefore common to assign positions, debuff behavior, add order etc in advance to try and reduce the amount of "reaction" you needed to do as much as possible.
But that's starting to ramble. Bottom line is: Yes, you can try to tune mechanic difficulty via reaction times, but it's iffy.