The situation with Serpent 55 and 52 resembles the situation with SeaMonkey and BNav. Of each pair, the former browsers are generally better, but are no longer actively maintained by their original creators.
With Serpent 55 (and a few other browsers in the same situation, such as FF 45 ESR), @roytam1 does what he can, importing fixes from ArcticFox and other sources to keep them as up-to-date as practical. But there are no weekly updates coming from "upstream;" that's why updates to these browsers occur irregularly.
I too happen to prefer Serpent 55; for me the main difference is slightly better support of WebExtension-style add-ons, particularly the Multi-Account Containers add-on. I don't mind getting less frequent updates in exchange.