No, you're correct. Just about every engine has its own lifecycle.
Some, designed for linehaul over the road multi trailer work might have an industry expectation of 1.4 million km, but I've seen new model series with problems only achieving 650, 000 in their hundreds and other new modesl series coming off the line and achieving 2 million km.
Going down to smaller diesel engines I've seen two different diesel engines going into the same application with one achieving a consistent 60,000 - 80,000 life cycle under the load and a competitor engine achievingf a consistent 250,000 and in later versions getting out to 800,000. km.
Nothing beats costing each vehicle (engine) separately and totalling the invoices to bring you down to earth.