Weak points on an H? There may be some, but the difference between an H's weak points and strong points are like comparing the durability of something that will never wear out to something that will wear out every 50 years or so. They are tough machines. From a restoration standpoint, the weakest parts and the hardest to find in my opinion are the sheet metal parts. Even then, with so many tractors made, these are still fairly easy to get.
There are some tractors that have real weak points (like the front steering casting on a Cub), but to my knowledge, as the other post indicates, the only potential problems are caused by neglect, abuse, and normal wear and tear.
A close inspection for missing parts, damaged parts, rusted out sheetmetal, worn tires, strange noises, and a good compression check is what is needed to prevent a bad purchase.