The Reach is an interesting region. It’s the most fertile and populous of all the Seven Kingdoms, the second richest (only losing to the Westerlands) and the home of Chivalry.
As we see in “A Clash of Kings”, they can field 80.000 men (without House Florent, one of the most powerful House in the Reach) and probably have no problem in maintaining this large army. It represents almost the double of most of the regions of Westeros can field and the Reach also has the highest number of knights.
So… where are the disadvantages?
1- They lack natural defenses
2- They are surrounded by several hostile regions (Kingdoms, depending on the era)
3- I read somewhere that the vassals of the Tyrell are not so loyal and don’t gather their hole strength for their liege lord (except for Renly, because he was charismatic), but I don’t know if this affirmation is true.
But, for me, that is not enough reason to justify how overpowered the Reach is.
It is my first post. I apologize if I wrote something wrong or forgot any important detail. I am also foreign and it’s not my mother language, then since now I apologize any huge mistake.