I'm not too sure I agree with that. I agree that it's very easy to kill an engine be it petrol or diesel, but with a bit of knowledge, patience and proper measurement tools, one can modify an engine to make more performance. In the case of older engines, you will almost always smoke more unless you raise the boost pressure. Smoke and EGT are related, so if you're smoking a lot, rest assured that you'll burn a valve seat or turbo in short order.
Oh and I've seen a bunch of chipped modern EFI engines kick the bucket as well. As in a mechanical indirect fuel injected diesel, bad tuning will kill it off as well.