Nyos is the only one I have used for Mg, but I find it pretty simple and straight forward. (Also use Nyos for Ca and Salifert for alkalinity. Wasn't a fan of the Red Sea alkalinity titration as it was too faint)
With the titration kits I always question whether it is done when the color begins to change but holds inbetween the beginning and final color or if it is done when it is solidly the final color. As in it starts off pink, begins transitioning to violet but shifts back to pink if you don't add any more titrant, then holds at violet, but another drop or two shifts to blue, which is supposed to be the final color. My problem is the wording states that the final color is blue but it is done when it begins to change blue (or whatever the final color is). Changing to violet is technically beginning to turn blue, but isn't actually turning blue.
(I know comsistancy is key, but when dealing with issues and trying to dose to fix a messed up salt mix, actually knowing what the parameters are is better. I'm also persnickity like that and can't justify testing if it is just a rough estimate to get close enough)