What are the challenges of implementing DNSSEC?

Question

Domain Name System

Question to the gurus: What is the difficulty of implementing DNSSEC at domain name registrars?

Answer 1

DPI, love for MITM unencrypted DNS traffic, that's it.

Answer 2

TLD zone holders need to support DNSSEC and provide APIs for adding records to domain registrars, who in turn would be willing to implement them. I understand that the main question is in the latter.
The experience of https://www.gandi.net shows that there are no problems with DNSSEC support in the presence of the above.