I have seen the cracks too - right through the fibreglass. The chainplate corrosion was more disconcerting.
I never had any problems with my 3 macs, 88 26-D 9926X, 2000 26X but I usually trailered them around and only had them in the water on on weekends, I would only keep them in the marina for 3 or 4 months Maxium per year. I would then tow them home and keep them under my deck for the witner and spring I never put them in Salt Water, but I did put them in the Polluted OHIo River..?/
I sold my 1999 26X to a neighbor. He towed it to the Florida Keys. Just as he was about to launch it, his wife noticed that the Right trailer tire had grinded a hole thru the hull. MACgregore Trailers are the worse. He bought a fibergalass reparir kit, patched the hole and Sailed it for 2 weeks. He had to have the Axel bearings and mount replacied before he drove back to Ohio.
MY 1988 26D Caugnt in rought water off the beach in ERie Pa with lightning bolts all around me, I decided to just run the boat up on th beach . a few large waves were impossible to steer and I hit the end of the break wall. The hull rode up over the massive rock. I made ti to the beach, Convince I had a hole. After the storm, we pushed the boat back out in the lake, I didnt see any water in the bildge.. I returned to the dock and put the boat on the trailer. The rock had scratched off about 1/4' of gel-coat about 10 ` long. from inside the bildge you could see light wher it was scratched. I bought iEpoxy and filled in the void. That was about 20 years ago. The boat is still Sailing. Last I heard it was on a small Inland Lake in NewYork..