I blame the low rankings of brook and franky on Oda and only oda. And not only because of design, but because jimbei just joined and feels around way longer then them because he got things to do since impel down, marineford, fishman island, WCI, wano. Franky made the less liked ship among the two and had some moments here and there like EL/W7 and segnor pink fight, that's it. Brook had well, almost nothing besides his BM thing before the wedding and when he smashed the carmel photo thing. And that is for people that joined like when, 12 years ago or even longer? They always felt like secondary straw hats, after the main crew formed in east blue.
Brook's had a ton of moments, he's just still super underrated. He gave us the iconic Binks's Sake, his fight with Zeo was one of the best of Fishman Island, and he was easily the MVP of Whole Cake Island, between the Poneglyph theft and what he did at the wedding. Aside from the Marineford Saga and Dressrosa (both of which he had to leave early) he's had a clear highlight for every major storyline since his introduction. I'm not sure what else Oda could have done with the guy.
Franky's definitely been underutilised and has a ton of wasted potential though.
The difficulty giving these two lategame recruits enough time to shine should bode poorly for a hypothetical Wano recruit, logically, but somehow it doesn't seem to have affected the two most popular cadidates.