Are full carbon forks less prone to failing from hidden defects than those with alloy steerers?
Short answer: no.
Longer answer: not always.
Proper answer (IMO), with full disclosure and explanation (of the reasoning):

In the presence of electrolyte, aluminium will galvanically corrode with carbon. First (old) versions of alu-carbon combinated parts were more often prone to this. Good sealing and carbon-aluminium interface design can help prevent this. Nowadays there's no rule of thumb - both for alu-carbon and for full carbon forks and frames. Some are well designed and well manufactured so they last long. Some are not.

Digression:
While with steel and aluminium manufacturers have come up with methods of designing and building frames so that even if they fail, cracks first show up on the outside (small, like a thin hair-thickness line, but visible with "naked eye"), and pre-ride checkup is all that's needed, with most carbon parts this is still not the case. Which is the scary part - parts failing catastrophically without prior warning. Don't want to do any fear mongering, modern good quality parts of renowned manufacturers are a safer bet and carbon is the frame material of the future IMO, just take a bit more caution when buying and using. Explained in digression 2 part.

Digression 2:
Testing carbon parts for flaws.
Tapping test can show some flaws, but not all. Ultrasonic scans are a safer way of determining faults. As is explained in this Tapping Test Youtube video.
Long, a bit tedious to watch, but very instructive video on carbon parts care, inspection and problem causes is this one: Cutting up expensive carbon frames.

Digression 3:
Care of carbon.
Having chosen a good quality part (whether frame, fork, bars, seatpost...) from a reputable manufacturer, care still needs to be taken. 
Using copper mounting paste for good grip with optimal pinch bolt tension when mounting parts (except on stems, for reasons explained nicely in this article - in the comment section I've had a very good and informative discussion with the author and learned a few things - Headset bearings don't last forever).
Beware of any paint scratches and chips - it is needed to protect moisture from entering. Moisture delaminates fibres from the inside, making carbon parts fail. It also protects the resin and epoxy glue from disintegration by sun rays (modern carbon parts have UV blocks added within the material as well, but to be on the safe side...). So paint touch ups using paint that won't harm the epoxy is a recommended. Metals will show visible rust signs on the outside, while carbon parts will start delaminating on the inside very often, without visible signs until it's very late.