Surely every single vertical or horizontal SRT manoeuvre is carried out with only one point of attachment.
We wear a single harness connected with a single 'D' M/R (or similar) and our chest ascender, top ascender security cord/rope, descender and cows tails are all attached to it.
I'm not aware of any accidents involving cows tails breaking on a via ferrata, or anywhere else for that matter, and would have thought the most likely failure on a via ferrata would be the carabiner when it is potentially abnormally loaded when it hits the next anchor supporting the cable in upwards of a FF5 fall!!
Neither am I aware of any harnesses, carabiners, ascenders or descenders failing during normal use.
I am aware of numerous incidents in the work place where the central 'D' M/R has not been closed correctly before use. In one case, a very experienced rope access technician (and caver) forgot to screw it up after lunch and after suffering a short fall over the edge of a 21 storey building (different storey) it completely opened up. He only survived the fall because the threaded section of his M/R jammed into his cow's tail rope and somehow jammed into the harness attachment and held. He was very lucky and quickly made himself safe. This was in the days before the modern industrial harnesses and before IRATA was formed. We always used 2 x M/R's after this incident.
In the workplace M/R's are generally considered to be for where a more permanent attachment is required, e.g., scaffolders full body harness and sternal attached energy absorbing lanyard. Most of the new Petzl industrial harnesses do not require any M/R's to make ascender or other such attachments.
Nearly all the fatalities in the rope access industry have been caused by the working rope being cut due to a sharp edge and either the back-up device not working or the back-up rope also failing. Appropriately placed re-belays and deviations should be a suitable control measure to minimise this hazard in a cave.
I don't remember entanglements being a reason the HSE accepted the outdoor pursuits industry could use a single rope, but there were many good reasons. I sat on a HSE work at height committee when the 2007 amendment to the Work at Height Regs. came out and the person who wrote it (who headed up our committee) made it very clear that if the failure of a single point of attachment resulted in a serious accident or worse, the HSE would throw the book at them and likely result in a much smaller outdoor pursuits industry due to the likely significantly increased insurance premiums that would follow.
I've been using a Petzl Omni Triact since they first came out as my central attachment and an OK Oval Triact for my Stop and have never had any issues with them at all. They've certainly had plenty of Derbyshire mud plastered all over them. I always give them a good clean and a thorough examination after each trip. In industry a triple action carabiner connector would always be regarded as the safest.
Double action twist lock connectors are the ones to avoid. I've seen plenty of these come undone during use and there is evidence of them failing catastrophically, as was the case with the Noel Edmunds incident many years ago.
A 'Thorough Examination' of cow's tails should always include untying the knots. If they are going to fail at all it will always be at the knot.
Depending on how much caving I'm doing I would usually change all my textiles out every 3 or so years. Metal items (subject to a successful thorough examination) are good forever.
I use a Petzl Dual Connect Adjust lanyard instead of traditional cow's tails with Sm'D screw gate carabiners on the ends of them. I rarely screw them up but occasionally appreciate the extra security they can provide. It only takes a second to screw them up.
I would never consider using a snap gate for rigging though. I've seen on numerous occasions carabiners and M/R's being twisted in the anchor at a re-belay when somebody climbs above it. I've done that myself on more than one occasion. It would be really easy for a single action connector to disconnect itself!!
As for the extra weight of screw gate or other double or triple action connectors being an argument for not choosing them, give over. You've probably got 100g of mud stuck to the bottom of each welly!!
Mark