His one attempt at something lasting and challenging, a giant bronze statue of his employer's father on a horse, was delayed many times, lying unfinished for years, and finally put to rest by the French invasion. The seventy tons of bronze set aside for it were appropriated for cannons, and the clay model was used as target practice by the victorious French.
The guy had professional ADD but despite this was one of the best artist and engineer of his time.
Plus, the Last Supper is a huge fresco, not just "a painting", which from concept to execution took several years.
He didn't build the canals, but he did make several observations and proposals for improvements which were taken up by other engineers after his death. These attempts were riddled with problems, though, and the work eventually terminated (I inferred unsuccessfully).