[timdorr]

There's also GDPR/CAN-SPAM/CASL/etc compliance and abuse handling. Those can be costly in terms of time, money, and access.

[lucideer]

These can be costly if you're using Mailchimp et al. Mailchimp's compliance features only handle compliance within the Mailchimp lifecycle, not at the entrypoints. The primary part being subscription management. Using Mailchimp doesn't guarantee compliance.