The disadvantage on geography makes Martell impossible to win by occupying 6 cities or strongholds. Tyrell can attack your city (Starfall) via sea transportation at any time. However, you have to march a long way if you want to attack Tyrell. Though you may take control of Storm End, I don't think you can easily defeat Baratheon in ShipBreaker because the support from BlackWater Bay can never be raided.
Furthermore, the total combat strenth of Martell's house card is 15 only. ( Greyjoy is 19, Lannister, Stark and Baratheon is 18, Tyrell is 16) In addition, the others have some hourse cards with powerful text ability that Martell don't have. The most important thing is that Martell hasn't a house card which can ignore opponent's text ability to against Tyrell and Baratheon.
Any tactics or ideas for Martell?