SH's aren't gonna stay beneath Doffy family forever. Franky may not be stronger than or as strong as Vergo now but he should be a lot closer to him than what you seem to think. SH's rapidly grow stronger in accordance with the demand of the plot. If Franky is able to beat some high-ranking officer in a Yonko crew that means he grew stronger, not that his enemy is weaker than a guy who was stronger than Franky 5 arc ago.