wpForo registration and login form are the same WP forms with different design. They use the same function on actions. I recommend change wpForo user Registration mode. Navigate in Dashboard to Forums > Settings > Features and enable  "Enable User Registration email confirmation" option.Then delete cache logout and test it again.

Also make sure the CPATCHA plugin is enabled for login and register forms in the plugin settings.

I've now tried "Fast Secure reCAPTCHA" by the same developer, same thing: captcha doesn't show up in wpforo login/register, but shows up in WP screens.

Actually, SI Captcha was working until 3.0.0.0 update (the captcha image showed up), but now it doesn't in wpforo. I assume the plugin incorrectly hooks (or does whatever WP magic) to inject the captcha, and fails to work for wpforo, but I'm just guessing here.

Email confirmation is enabled (was from the first time wpforo added support), but it doesn't seem to be effective enough to stop spam registrations (once I got a wave of like 5-10 in a row).

Please try others, or deactivate all plugins, delete cache and test it again. This maybe plugin conflict.

Do you have any recommendation for plugins that are known and tested to work? SI Captcha was working, it's the latest version that doesn't.

However, I'd very much be the happiest if I wouldn't have to resort to plugins and wpforo had reCaptcha support built-in, like Contact Form 7 does. Do you have any plans to add reCaptcha support?

I switched to Captcha Bank
It is simpler and works for registration in the forum and also for comments in the blog