The issue JBs have when being compared to Serpen is Vinto.

Serpen still has faster vintos, and it still has Vintos with more range. On top of that, in content that doesn't have damage resist on enemies, but also has damage cap, when it comes to damage per vinto, serpen still reaches cap and anything stronger becomes a "waste" while also putting out less vintos than Serpen. The speed and damage cap issues create a serious bottleneck for JB DPS. And since they've yet to buff the rest of JBs PAs to have good enough output where vinto all the time from serpen is no longer the best, there is still a serious argument that Serpen is still more practical overall (not all the time, but more than enough to deter you from investing in Cras JB)