I use acme.sh for certs on my personal server and was a little surprised when it started using ZeroSSL by default. Despite being more "corporate" I decided to roll with it and it's worked just fine.
None. Big tech intentionally made Let's Encrypt a single point of giant failure.
> And in case none exists, what does it take to build one?
A new Internet and Web standards stack. The whole problem is self-imposed -- we could have published self-signed Ed25519 keys on the DNS instead, and the result would be more secure than whatever it is we have now.
Do you remember the early days of SSL certificates? It took an act of god just to get a certificate: verification rituals like faxing corporate paper work, phone calls, manually reissuing certs because someone forgot the "www", forgotten renewals...
This is Stockholm syndrome. You were taken hostage and beaten and raped. Now the rape has stopped and you get gruel on schedule, and you're enamored with your captor.
Requirements: free, available to everyone, automation friendly, issues certificates that are actually considered trustworthy by other parties.