By the looks of the "squat" on the Truck rear end/tyres, the axle. suspension &/or tyre loadings are being pushed to or past their limits.....& possible shock loading on the truck body/mounts/suspension may prove ti have interesting "impacts" over time.
Before selling a Sleipner to a customer, we always make sure, that the customer has compatible vehicles for towing the excavator - Sleipner combination. In a usual situation 65 - 75 % of the excavator weight is on the Sleipner units, and the remaining 25-35 % on the truck.
A quick calculation for the combination posted by Nige:
Ex3600 - 360 tonnes
Cat 789C - Payload capacity 177 tonnes
Material on truck: approximately 20 % of payload = 35 tonnes
Weight on Sleipner: 360 x 0,65 = 234 tonnes
Weight on Truck: 360 x 0,35 + 35 = 161 tonnes
There should be no problem to use this combination even in a longer period of time.