I don't know which software this other forum uses, but XenForo recommends Cloudflare as the best way to keep a forum secure without CAPTCHA. When I set up this forum, I created a Cloudflare account. After hearing from a couple of people who said Cloudflare wasn't letting them in, I tried registering a new account in my usual Safari browser and had no trouble, but when I tried it on another browser, I don't remember which, it wouldn't allow me in either, so I deactivated Cloudflare.
I review websites for a living in my web directory job, and every day I come across sites I cannot access because of Cloudflare. I'm not doing anything nefarious, so it is being overly protective, which is excluding legitimate visitors. Generally, the website owners don't even know who is being excluded.