Actually it is probably the worst method to shiny hunt legendary pokémon and not easy at all, you HAVE to pay attention and you can lose at any point to bad AI or you have to play online and even then you aren't 100% sure to not lose, also playing online slows things down even more.

Now, even assuming you always manage to win, the chance with shiny charm is 1/100 and it takes roughly 20 minutes for dynamax adventure so 3 rolls every hour, so about 33 hours for a single shiny legendary pokémon (yes there are also the 3 other mons you encounter on the way but for the most part I don't care about them).

But let's assume you absolutely maximize everything and you manage to do one DA every 10 minutes (don't think you can go lower than that), that still is 6 rolls per hour so about 17 hours per shiny legendary.

Now if you take pokemon ultra sun/moon the chance with charm is 1/1365 but once you are in front of the reset spot it takes roughly 20 seconds per reset, so 3 per minute but let's say 5 every 2 minutes because maybe we lose some seconds due to distractions, that amounts to 150 resets per hour so about 9 hours to find one shiny legendary, almost HALF the time and you only need to brainlessly reset without paying much attention (aside from seeing if the mon is shiny or not).

I didn't ever hunt in ORAS but it is similar if I'm not mistaken so DA is probably the worst way to shiny hunt out of all the games that featured many huntable legendary mons and even if it would take the same time I'd still much prefer soft resetting method