I'm quite confident on why they don't want to share their data and/or methodology but there could be legitimate reasons and I want to be open to those.
Regardless, without that information, we can only evaluate them based on how rigorus they've been in the past:
Are the researchers and organizations involved known for effective data collection and solid analysis?
>but there could be legitimate reasons and I want to be open to those
Then they the should just say what those reasons are. What's there to hide?
You can't expect to be opaque and then the public to blindly trust you simply on the basis that you call yourself experts.