What Are the Biggest Challenges in Onboarding Developers?

What are the main challenges you face when onboarding new developers? I’m mapping out how teams handle key aspects like knowledge sharing, defining team culture, and introducing coding standards. Insights from Engineering Managers / CTOs, etc or anyone frequently onboarding devs would be incredibly valuable. I’m especially interested in understanding where things tend to break down, but also what strategies or tools are working well for you. Thanks in advance for your input!