Hi EKA Wrecker,
In my past I did a lot of work with Foden recovery vehicles, and also did all the testing of them for our area at the time (crane, winches, hydraulics, etc). I carried out overload testing when required, to ascertain that the safety systems were working and can tell you that it is quite a stable machine, but obviously if using it on soft ground you would have to be aware of legs sinking, etc. I cannot remember overload weights, but may come across them and post for oyur interest.
Your mention of instability, reminds me that the actual vehicle when driving, has a maximum inclination of 27 degrees if my memory serves me. I was involved with these from date into service until 1996, so a little while ago now!