The failure of these plastic zippers is very widely documented. It seems that they come in two flavours: the ones that die somewhere between 30 and 70 dives, and the ones that last for perhaps 300+ dives.
The first ones on the market, the T-zips, failed "en masse" to the point where most suit manufacturers (including SANTI, DUI and Bare that I personally know of) abandoned them in favour of YYK plastic zippers. I can personally attest to the crappiness (is that a word?) of the T-zips, having replaced them three times, thankfully, always under warranty.
I'm currently using a YYK plastic in a new suit with perhaps 150 dives on it. It hasn't failed, but it's fussy af and I find I need to lube the dock before literally every dive. This leads me to believe that it's the lube that is doing the sealing, not the zipper. I think I can honestly say that it is never 100% dry. The degree of dampness varies widely for some reason. Literally everyone here dives dry, and my experience is pretty typical, if not better than most.
As for why the suit manufacturers use them, I think the answer is simple: they're cheap.
All of this is in contrast to my previous 30 years of diving dry with a brass (which is "metal") zipper where I NEVER had to replace a zipper. 700, 800 dives was typical before I would replace the suit, and hence the zipper too.
@CAPTAIN SINBAD I agree with you. I love the plastic in a front-zip suit because of it's flexibility, but if/when this suit fails, I'll replace it with a brass one.