I have no experience with this particular vendor/brand but the adapters are not complex. If you're concerned, verify that it has 11VSB before you plug it into the motherboard. Otherwise it's entirely passive (passing through PS_ON#, PWR_OK, and 12V)

> If so, I wonder if there is a hit in efficiency because of the required step-up / step-down converters in that adapter.

It should be quite nominal. The boost converters have a very high efficiency (>85%) and standby power is typically below 10W max.