I feel that he's in a good spot, I don't see him as a failure. He was the one who understood WB the best and went on to fullfil his wishes.
The WB Pirates were unlike any other crew, they funcioned more like a family (funnily, much better than an actual family like the Charlotte's). WB's main goal was protecting his territories and adding more "sons" to his family (any one who was in a dire situation could join him). I think that he basically wanted to eradicate all the Sphinxes (poor countries) around the world. Maybe after meeting Ace he added a side quest to nurture him to become the next Pirate King (fulfilling his rival's -- and friend's -- wish), but he ultimately couldn't do it (not due to his fault, but due to Ace's boldness). It's worth remembering that Roger told him about the secrets of the world when he last met WB.
Marco tried to follow through, but as he was unable to defeat Blackbeard, his best course of action would be preserving WB's family the best he could; so he retreated and went on to directly help WB's hometown. He only helped Luffy because it was WB's last wish, he wanted to preserve his life in Marineford for him to carry on with Roger's (and Ace's) will.
In terms of strength, I'd put him in the upper echelon of the YCs, he's IMO one of the strongest YC1's. Marco's above Shiryu, Katakuri and King IMO, despite not even showing CoC or any advanced haki form.