The rate limits have been changed to 20 certificates per domain per week recently.
The registrations/IP rate limits aren't really a problem - WordPress could, in theory, run their entire Let's Encrypt infrastructure using one registration (account).
Pending authorizations shouldn't be much of an issue given that all custom domains are CNAMEs pointing to their servers, so they should be able to solve all challenges.
(By the way: If you're building a large integration, Let's Encrypt can change the rate limits for you.)
The registrations/IP rate limits aren't really a problem - WordPress could, in theory, run their entire Let's Encrypt infrastructure using one registration (account).
Pending authorizations shouldn't be much of an issue given that all custom domains are CNAMEs pointing to their servers, so they should be able to solve all challenges.
(By the way: If you're building a large integration, Let's Encrypt can change the rate limits for you.)