So you mean that only the most recent data would then be lost?
I would be afraid that the metadata might then no longer be correct.
How would it be any different from a random power outage without a SLOG, in which "sync" writes were using the ZIL area of the pool itself (located on the data drives)?
This means power outages would be causing a slew of corrupted pools and metadata.
ZFS is copy-on-write, so it's "either-or". Either you have it or you don't. Either the record was written and the pointer updated/created, or it wasn't. This includes metadata records as well.
A SLOG is just leveraging the performance of a faster device (compared to the devices that comprise your data pool). So instead of using a region on the data drives of the pool itself (ZIL), your
sync writes use a separate (faster) device, which is your SLOG. Same issue, with or without one, in regards to powerloss or failure. You lose the most recently written data. The only difference is if your SLOG has built-in powerloss protection, which can "save" these recent writes; and upon rebooting the system, they will be read and written to the data drives of the pool.
Someone more versed in the nitty gritty can better explain this.