As with cars, what used to be go to rock solid reliable makes arn’t necessarily as good as they once were whilst one or two others who might have lost sight of the ball for a while are making a comeback.
Much depends on the type of work ie a parcel trunker on light motorway work in theory the vehicle should go on for ever, a full freighted bulk carrier on varied terrain the engine might need a rebuild at 800k.
How the vehicle is looked after re good preventative maintenance, maybe more frequent servicing than the typical 5 year leaseplan where the engine sees new oil every 100k kms.
Mechanical sympathy from the driver.
Buy new and you are in control of all the above, buy used and you really don’t know what the vehicle has been subject to over its life.
If i was buying new or nearly new from my most recent experiences it would be Scania or MAN for reliability, 10 or more years ago 3 other makes would have been considerable.