Which feature of AWS CloudFormation you find the most difficult to wrap your head around?

Hi there!
I am by no means the greatest of all when it comes down to CloudFormation (CFN) yet it's a service that I interact mostly on a daily basis and I've had the opportunity to delve into it for some time.

Here I come, I'd like to pay it forward to the community and create a couple posts related to it.

Is it Lambda-Backed Custom Resources?
Cloud Development Kit (CDK)?
Automate CFN template deployments?
Deploy stacks to multiple accounts?
General CloudFormation knowledge?

Please do let me know in the comments and I will try to come up with weekly posts about it.