What makes it particularly ambitious? The strait of Messina is two miles across, and I don't think that even cracks the top 100 of the world's longest bridges.
The current longest is in Turkey at 2023 meter.
Each of the pylons of the Messina Bridge will be around 400 meters tall. Which is taller than the Empire State Building.
The strait is too deep, with too much current and seismic activity to place the pylons in the water. So they have to be on the shore, as I understand it.
The strong presence of organized crime in the area also makes a lot of people uneasy about the whole deal, but that's not a technical issue.
> "...building a suspension bridge of this scale poses significant engineering challenges. The Strait of Messina is known for strong winds, seismic activity, and deep waters, all of which complicate construction and long-term stability. Engineers will need to ensure the structure can withstand earthquakes, which aren't unheard of in the region, while addressing corrosion from the salty marine environment."
-- https://www.iflscience.com/worlds-longest-suspension-bridge-...
I remember reading an article, posted here on HN, that went into much more depth about why this was all unusually challenging, but I haven't found it again.