The Nacra 20 is a pretty solid boat and I have not really seen any common flaws in the hulls are rigging. There have been some changes to the rigging as the boat matured.

there was an issue with the 'old-style' trampoline that slid into milled grooves in the side of the hull. In reality, the bolt ropes on replacement trampolines were too small which resulted in the trampoline pulling out from the hull. Old original trampolines rarely suffered from this problem. You can send an original trampoline back to be updated with a larger bolt rope and side lacing for about $150 US.

Other than that, look for corrosion on the beams (particularly around the mast post), closely inspect the mast for any cracking (I've never seen any but it is a carbon mast), and like any other catamaran check the stiffness of the decks.