No idea what "focus slip" is, and have no idea who would say they're not stable. They're fine. Lock down tight and strong timber and Aluminium construction. I have the oldest copy (prototype) and have put hundreds of rolls through in places as diverse as The Scottish Highlands in Feb through desert trips with 43+ degree heat and in tropical jungles. It never missed a beat.
There is quite a bit on these over at Large Format forum.