You can derate them to 3.5 tonnes Chris, so no issue there, just that the payload is lower.
They are becoming popular in many markets because of their lower purchase prices, list of optional extras, and much lower running costs then the Unimog, Spain have a number as fire engines in inhospitable areas, and forestry vehicles, and many other vehicles are being sold worldwide.
Their simple turbo diesel is cheap to run, easy to repair due to a worldwide dealer network, and spares and service parts are easily accessible at much lower prices than Mercedes items.