Paul, I have not tried the spinlocks, only heard that the plastic cam wore out over time. But there are lots of teams using spinlocks on everything from mainsheets to downhaul cleats. Looking forward to hearing more about eventual wear issues on these units over some time.

About plastic and cleats: We often sail two up on a Tornado with crew weight of 190Kg. In windy conditions we have broken two Harken cleats and one Holt on the spi halyard. All was oversized for the load (6mm line + 8mm hose = 8mm line thickness) but made of plastic. We have ended up with an 20 year old alu cleat looted from our first boat. We had the same experience with the roller bearings in the swivel base. After two replacement wheels/bearings, we fitted a stainless steel block inside the swivel base and never looked back.
If you want to put high loads on a cam cleat, I think line diameter is critical. Also, if one of the cams are mis-aligned when you cleat the line, the device will break. Both cams need to engage, if one of the cams stay in it's 'starting position' it will break the retaining pin and invert.

I guess your experiences might be due to not having a perfect match between your halyards diameter and the cleat. As I wrote above, we had to sleeve a 8mm hose over our 6mm halyard to make it stay in the cleat.