The squaring planet in a T-square mediates the opposition between the other two planets. The difficulty is that square aspects require adjustment: energy doesn't flow as easily when it has to change course. When a T-square becomes calcified, there needs to be a way out, so to speak. It's frequently found at the point opposing the squaring planet.
Grand trines seem like very nice things, and sometimes they are. The three planets or configurations on each point are usually in the same element and so they communicate very well. The difficulty is that the grand trine can take over the whole horoscope, to the exclusion of everything else, because it's such an easy place to focus one's attention and resources. Needs and behaviours interrelate and reinforce each other. If neither the Sun nor Moon is involved in the grand trine, the structure can seem to operate outside of the principal needs of the individual.
The most important thing is not to fall into the trap of dissecting T-squares and grand trines as though they're just assemblages of parts. They need to be seen as wholes in themselves. Then, analysis flows easily.