Although I like doing things sync'd for WT, there are a few things that I wouldn't dare to do sync'd, such as anything in the Second and Final Coils.
To be honest, it really depends on the average player skill of your data centre that makes it harder or easier to do content sync'd.
For example, I have two characters, one on JP DC Elemental and one on NA DC Aether. Most of the time, we can usually quickly queue at prime time and one shot all the primals sync'd on Elemental while on Aether it takes a long time to queue even at prime time and it takes multiple wipes if people don't know what they're doing.
People take the path of least resistance, and unsyncing content is that path.

If you couldn't unsync, Second and Final Coil would 99% be untouched and not even new player bonuses can save it.