WeBuilt, the general contractor, is one of the biggest building companies in the world. They even built a part of Panama Channel expansion.
Why do you think that they are mafiosi?
Personally I don't know if this bridge would be useful or not, but I think that refuse to building it just because mafia risk it's a stupid reason. Italy has a strong law and control system to prevent that and I remembered exactly the same argument about Bologna-Firenze high speed train tunnel and highways that after almost 20 years are successful infrastructures.
Ndrangheta is a huge criminal organization with ties to politics and a lot of economic power and it is based in Calabria.
The current government actively proposed special antimafia rules for checking contractors and subcontractors for the bridge which, IMHO, reinforce the idea that it is a gift to mafia. Luckily the proposal was rejected by the president https://en.ilsole24ore.com/art/strait-bridge-no-derogation-a...