In your experience, what are the main challenges when trying to get users to enroll additional devices? For example, is the enrollment process too complicated, or do users simply not know they can register their personal devices? I’d love to hear any specific examples, hurdles you’ve encountered, or strategies that have worked (or failed) in your organization. 

The most common reason I've heard for not enrolling is not wanting to grant that level of security to the company as it would limit their functionality. On devices that are registered, restrictions are put in place like not being able to install some applications, the entire device can be wiped, and password requirements are extensive.

We have various solutions such as security policies but also access restrictions for BYOD devices.
If you have a personal smartphone, you can get an isolated work environment that allows certain apps to connect to the corporate network.

We have security policies in place that restrict access to corporate data unless the personal device is enrolled and our data can be managed / controlled by IT.