Well, that was fast.
I was watching a video online of a shiny Guzzlord encounter, when I looked back over at my 3DS, and encountered a shiny Guzzlord myself.
I took a break from SR'ing yesterday evening due to my 3DS freezing repeatedly, but I continued the hunt this morning, and I found the shiny rather quickly. (And no, my 3DS has not frozen today, but I am still concerned that it might again.)
I have done maybe about 5 hours or so of SR'ing total for this shiny. It seems that after that week long shiny Cobalion hunt I completed yesterday morning, I had some returning good luck for this US version.
Anyways, it only took two Beast Balls to catch this thing. I knew that it could use Thrash (which it did on turn one) and get confused, so I wanted to catch it before that happened.
Adamant Synchronize failed again (seriously...?), and this shiny is Relaxed. Oh well. It has an IV spread of x/31/x/31/31/x. Fine, I suppose.
I now have only one more Ultra Sun UB to SR for shiny, which is Xurkitree. That is one that I would really like, and I would want Synchronize to work on that more than the others.
I then may play through my Ultra Moon version to SR for version-exclusive UB's and legendaries there. I have been thinking about doing that soon anyways.
Good luck to everyone else on your hunts!
EDIT: Well, I thought there was more space between this post and my previous one. Oh well.
EDIT 2: So, I was training some of my Pokémon in a S.O.S chain of Chanseys in Poni Plains when probably over the 70 encounter threshold for higher shiny chances (wait for it...) I encountered a shiny Chansey (there is the pun).
...NOW Synchronize works? She is Modest with an IV spread of 31/x/31/x/31/31. How interesting. I wonder what I can do with this thing.