A bit of background, mostly in regard to the Spanish flota galleons or Manila galleons, which are probably the best-recorded of all the various historical 'treasure fleets'
First, far more Treasure Galleons were lost to storms - especially hurricanes in the Caribbean - than to enemy action.
Second, when ships were lost to an enemy, it was very, very rarely in the open ocean. Ships were just too hard to find in the open ocean, so most battles took place in or just offshore from a port or a meeting/assembly point for the treasure fleets, or in the port itself as part of an attack on the city. In fact, only one Spanish treasure fleet was ever destroyed/captured by an enemy - the Dutch admiral Piet Hein took an entire fleet as it approached Spain from the New World, but he used an entire Dutch naval fleet to do it.
Third, there was a distinct difference in real life (which is not made in the game) between individual Galleons and 'Treasure' Fleets of Galleons. The treasure fleets that ran between the Caribbean and Spain were largely composed of special 'flota galleons' of about 600 tons each and carrying 20 - 40 cannon, and including frigates and other warships as escorts. As posted above, only one such fleet was ever captured by an enemy in over 200 years.
The Manila Galleons that moved between the Spanish New World and Asia, on the other hand, usually sailed alone, and were much bigger ships - averaging 2000 tons. They were also almost all built in either Manila or Mexico rather than Spain, and sailed with 'native' crews from the Americas or Asia. These are the ships that sailed from America with up to 2,000,000 silver coins each bound for the Chinese market, and returned loaded with silk and porcelain: each ship carrying the modern dollar value of almost an entire fleet's cargo.
In game terms, since all of the Legacy ships are referred to as 'fleets' we have to assume the game is bunching individual galleons and fleets of flota galleons together in one unit. That being true, they should be very hard to capture or destroy - historically, it only happened once, and very few individual ships were ever taken out of the fleets, either - whereas the majority of 'treasure wrecks' found are of ships that went down in storms, both in or out of fleets.
Privateers, by the way, whether full-blown Pirates or hiding behind letters of marque, never took a treasure fleet and rarely even attacked one - they stuck to individual ships or very rarely raiding the ports they sailed from or to. In game terms, then, 'Pirates; would be an annoying nuisance but no major threat to the Treasure Fleets.