The biggest challenge comes from cross-functional partners (engineering, sales, marketing) who may not have prior experience partnering with PMs and aren't bought in to introducing the function. The real value of a PM is to have a dedicated resource thinking about what the highest-leverage activities fro the team are in service of the broader business goals and user problems, but in many orgs that role has been played piecemeal by various folks and solidifying into a formal function can feel premature / overkill / threatening.
Additionally, when introducing PMs into a company that hasn't had the role before, you also have an external challenge of creating enough of a brand to attract talent to hire.