I've never seen a fastex buckle break either, but the "quick adjust" harness feature isn't quite as helpful as it first seems, at least in my experience.Im new to this, just got a Dive Rite HydroLite which has adjustable harness. I can't see the point of fixed harness. It seems harder to size initially and get in and out of. It won't work well with different wetsuit thickness (skin to 7mm) The only advantage I see is "points of failure". While in Cave diving I can see that as a major deal, for most recreational warm water diving, so what if a buckle opens up? I can simply reach over and clasp it back. I've had to open and tighten weight belts underwater before and it's never a big issue. So for recreational warm water diving, why would I use continuous webbing
Benefit to everyone is consistency and simplicity. Yes, it does take some time to set initially, but then its set and you're done.
A properly fitted one piece harness has a decent amount of slack in the shoulder straps and not difficult at all to don. Then when the waist is cinched, pulling the crotch strap tight, it also pulls a lot of the of the slack out of the shoulder straps and your plate sits at a fixed position on your back. From there, you can fine tune position by adding a little extra length to the shoulder straps and tightening the crotch strap, and vice versa. Once you've got that set exactly where you like it, its pretty darn consistent from dive to dive. Your trim doesn't change based on variations in plate/tank position, and your valve and d-rings are always right where you expect them. It makes donning your gear a simple on/off operation and with no futzing around with harness length on a rocking boat.
Caveat - if I'm diving in cold water, I'm in a drysuit with a steel plate and harness length doesn't need to ever change. If I'm diving in the ocean wearing just a rashguard, i'm diving a different aluminum plate, and yes I do have the shoulder harness set slightly shorter on that one.
I don't think your life is going to change one way or the other, but a one-piece harness isn't just a safety thing for tech divers, to me it seems much simpler and easier and I'd recommend anyone with decent shoulder range of motion give it a try for a few dives before adding a bunch of unnecessary hardware to their rig.