The problem is that Let's Encrypt enforces rate limits on how frequently you can request new certificates. According to the documentation[1], the limit averages out to one certificate (which can include up to 100 hostnames) per roughly 8 hours, per domain.

That's probably good enough for almost everyone who uses hostnames to represent physical machines or services. But it's totally unusable if you want to create certificates on the fly in response to user signups.

[1]: https://letsencrypt.org/docs/rate-limits/

This is the real issue with not allowing wildcard certs. If the rate limits were something that just cut in at abuse-levels, there'd be no need for them. Sites with per-user subdomains (to get security features like cookie isolation, break same-origin effects, etc) run into the limits really quickly, and must currently rely on a wildcard.

I've been calling this "the tumblr scenario", but people seem to think there's a way around it without just using another CA.

I deploy all the different little code projects I make to different domains and I've hit the Let's Encrypt rate limit. If (like me) you waited a couple months to secure these little side projects, you'll hit the rate limit too. That being said, I just waited a week -- now everything is secure.

I'd imagine you might be able to reach out to them in that case.

Even with good tooling the current restrictions on Let's Encrypt make it impossible, you can batch many subdomains into 1 request however if you're requesting subdomains by customer username or similar, you can't exactly wait around to batch them. And you can only make a maximum of 5 requests per week, so unless you're incredibly tiny it's non-viable.

I don't understand why Let's Encrypt can't consider validation of the root domain good enough to produce a wildcard. Email at the root domain is what most providers use, not exactly much worse.

EDIT: It's now 20 per domain per week, better but still not viable for even a mid scale operation. A single wildcard is a much nicer and easier to maintain solution in any case.