"Benefit" is to "pro" as what is to "con"?
What word would best suit this analogy?
Drawback is the first word that comes to mind. Other possibilities listed by Wiktionary are disadvantage, hindrance, encumbrance, nuisance, obstacle, or detriment. (Listed more-or-less in decreasing order of relevance, at least in my opinion.)
Detriment?
I'm not sure exactly how benefit should relate to pro, but as pro and con are opposites, I would consider detriment to be appropriate.
Merriam-Webster also lists: disadvantage, drawback, encumbrance, hindrance, impediment, minus as antonyms for benefit.