There's a very pertinent historical reference point: Ferdinand de Lesseps. He built the Suez Canal from 1859 to 1869, a task which was considered by many to be impossible or at least impracticable [1]. After achieving resounding success, he then turned to Panama to try building the Panama Canal. In this venture, he turned out to have several very wrong ideas (it's not feasible to build a sea-level canal in Panama), and the French effort failed so hard it actually brought down the French government [2]. He was successful in the first venture but for different reasons than initially assumed (namely, the Suez Canal isn't all that challenging from an engineering perspective; it's just capital-intensive), and his failure in the second venture comes from misunderstanding the problems.

[1] There had been canals in the past that traveled from the Red Sea to the Nile delta, but these invariably silted up rather quickly (the Nile was sediment-heavy, and wind-blown sand is quite common in the region). Thus there was historical evidence favoring the idea that the dredging maintenance fees would exceed the revenue from passing ships.

[2] Of course, this was the Third Republic, which is notorious for having provided series of weak, unstable governments replete with scandals that prompt new governments. Ironically, the Third Republic was the longest-lasting form of government France has had since the Ancien Régime.