There are some modern construction techniques that make me nervous:
One is pre-stressed concrete. This has replaced I-beams for highway bridges. The problem here is how do you inspect them? With steel I-beams you can use your eyes. With concrete maybe you need some kind of ultrasound equipment to check the tension cables? Do you trust those responsible for long-term maintenance to do it?
Another is engineered wood I-beams for houses. Basically these are floor joists made out of plywood. How long will they last? What happens if they get wet? If there is a fire the house is gone because the floor joists will be ruined by the water to put the fire out.
There were certainly mistakes in the past as well. One is building with cast-iron beams. They look nice, but they crack.