I finally got around to testing my pile of plextors. (like ~25 drives) the other day.
by far the biggest failure rate was from the PX-708A and PX-716A drives that i had piled up.
100% failure from the PX-708A (1/1 drives faulty)
50% failure rate from the PX-716A (3 of 6 were completely knackered)
but then again the one PX-716UF I have has been a really stable drive.
I'm, unfortunately, not surprised. I wasn't familiar with the durability issues of the 700-series drives until I purchased my troubled PX-716AL drives, but it's apparently a somewhat known issue. I noticed that someone added a warning to the Redump Wiki about these drives:
The Plextors in the 700 range that can read DVDs have firmware bugs and the lasers can go bad easily.
It's a shame, because the 716 was quite well received in its day, AFAIK. Even now, my barely working 716AL sometimes has more accurate C2 reporting than the 760 I use as my main ripper. I had multiple discs recently that exhibited no C2 errors but had defective sectors from the 760. The 716 reported C2 errors on the same sectors and successfully re-read. But, I think replacement lasers for the 755/760 are widely available, while nobody seems to be able to source one for the 716.