https://github.com/mise-plugins <-- First they say "Try to get your tool into aqua or see if it can be installed with the github backend, then it may be added to the mise registry", and then later they say "The rest of this doc is outdated and does not reflect the current state of preferring aqua/ubi.".
Overall there's too many ways to install things and it's not easy to add any of them. Asdf plugins were easy, but insecure (which could be fixed, but whatever). Everything else requires more research because it's more technical.
Only if they have a plugin that describes how to install them. Many popular tools are much more complex to install and set up than just downloading a binary and making it executable. For those you need to create a plugin for mise to be able to install them. Luckily, very often some other generous person has gone through all the trouble of learning how to make the plugin, going to the official repos, making a PR, and finally getting it merged. But if somebody hasn't done that already, it's painful (more painful than, say, an asdf plugin). It depends on the language, on the tool and system requirements, etc. Overall it's kind of a mess. Mise leaves you with the trouble of figuring all that out, rather than making some kind of convenience function to get the process started easily.
> Many popular tools are much more complex to install
I'm curious which dev tools you're using aren't installable with standard mise backends. 99% of dev tools I use don't require a plugin.
> (more painful than, say, an asdf plugin)
You can still use asdf plugins, I could use mise to install an asdf plugin right now with one line `mise use asdf:raimon49/asdf-hurl`. The mise registry is just a convenient list of aliases, even if it doesn't accept new asdf plugins, you don't need it to.
As Larry Wall said "make easy things easy and hard things possible"
For example there's also the GitHub backend which lets you install binaries from releases, no plugin needed at all.