Dorne is useless. Always has been, always will be. Their offensive power is the weakest in all of the Seven Kingdoms.
The only reason why they're regarded so highly is author-wanking on GRRM's part by making them the special sandgrain nation of Westeros, who avoid getting subjugated by three fully grown dragons, who avoid infamy for attacking while carrying a truce banner, who torture foreign royalty and get away with it scot-free... seriously, they're shit, and they'd have been cinders a long time ago if GRRM didn't make it otherwise.