They have good potential just cost more due to smaller injectors, 15deg hpop, no intercooler. What do you mean by high millage my old f250 had almost 300k and still ran strong put down 418rwhp and 837rwtq all stock internals. Strong point of the older 7.3 is all forged internals and they will last forever as long as maintance is done on them.
Things that go wrong are cracked fuel bowls, leaking pedestal o rings, rusted out oil pans and oil cooler o rings going out.