The issue is that people are comparing Fisher Tiger to Dragon because he was just an average pirate, he went there and saved many slaves in the land protected by admirals, without fearing anything or anyone. Dragon took 2 years, even though he is the most dangerous focking man in the world and Garp's son, he is certainly not weak. He possibly has Devil Fruit and Coc. And he neglected to save Ginny.
That's why Fisher Tiger was successful: it was a completely unexpected assault, and the people underestimated him because he was a Fishman. Taking a large force into Mary Geoise will just lead to losses.
The Revs 14 years ago were in no position to win a war in Mary Geoise, same as now.
Dragon has an entire army he's building, Fisher Tiger was desperate and felt like he had absolutely nothing left to lose. Fisher Tiger's goal was to free as many slaves as possible, Dragon's goal is totally different, to overthrow the WG.
This is what makes Dragon interesting, he's the complete opposite of Luffy and Garp, who kind of do as they please and rush ahead. Dragon arrives like a storm, out of nowhere, like them, but with more planning, IMO.
Like, Luffy never could have orchestrated the attack on Mary Geoise, let's be serious. I do think it's odd Dragon didn't participate with Sabo and the others, though.