Actually, what has happened to a few users(which is where I got some experience with this) is that you will lose data if you don't have a redundant ZIL and have an unexpected loss of power. When power is restored if your ZIL doesn't exist the pool will refuse to mount. It'll require you to do some parameter that basically kicks the missing/corrupt ZIL out of the pool, remove the incomplete transaction data, and continue on its way. So you'll lose the data in the ZIL when the power loss/kernel panic occurs. That' not usually much data, but.. read below.
I don't mean to thread hijack at all (its basically the same topic) but is sync really required on a system with redundant backplanes, controllers, PDU's, power supplies and backup generator power? If you are essentially 99.9999% sure it won't lose power?
That's for you to decide. There's no right or wrong answer, except for that 0.0001% chance that you'll still lose power and therefore lose data.
Imagine if your bank transferred $1000 to Apple for a computer you had just purchased, but because Apple thought that the 0.00001% chance was zero and decided to disable all sync and has no record of your purchase, but your bank gave Apple the money and Apple has your money but they have no way of proving what you ordered, if you got your order etc. You'd be a little pissed that you are now out $1000 because you can't prove anything.
ZFS is about having complete trust that the data either IS there or ISN'T there. There is no "might". When you do sync=disabled, you create a huge "might". It's your choice whether you like that "might" or not.
But think about this.. didn't you go to ZFS because you wanted faith that your data wasn't being corrupted? So why would you go to ZFS for that kind of faith, then deliberately destroy that faith? Sounds kind of stupid to go with ZFS, but then to disable sync. Either you care about your data(which is why you went to ZFS in the first place) or you can't figure out what your priorities are(in which case you need to walk away from the keyboard and mouse and get your priorities straight first).