When comparing FreeBSD vs Haiku, the Slant community recommends Haiku for most people. In the question“What are the best operating systems for advanced users?” Haiku is ranked 16th while FreeBSD is ranked 18th.
Specs
Ranked in these QuestionsQuestion Ranking
Pros
Pro Secure and stable
Pro Large repository of precompiled packages and highly customizable ports
Pro BSD license
Pro Easy to install
Pro Good documentation via the FreeBSD handbook
Pro Customizable
You can customize EVERYTHING as its an UNIX OS!
Pro Very fast
Pro Beta has been released
After about 6 years since the alpha version, beta has been released on Fri, 2018-09-28. Check here for release notes.
Pro Only need 512mb ram
Pro Stability
Pro Runs perfectly on old Hardware
Cons
Con Hardware support is lagging
Con Installing with "software source code ports" and software packages can not be mixed
Con Too much hardware is not supported
Con Not for everyday use as a Desktop
Con No one uses it
It's a very niche OS that no one uses.
Con Unfinished
It's still in beta and quite unstable. Making it unsuitable for developing applications of any kind.
Con Language support is terrible
Con Small community
It is important when developing to be familiar with tools that other developers use. You can make any utility in any language you feel like, but if it's in an esoteric language that no one can read targeting a small platform that no one uses, then it was just something you did as a hobbyist, not as a developer.
This is not to say that Haiku isn't a great operating system to hack around on. Just don't delude yourself into thinking you're doing it to get familiar with tools that you need to know to be a better developer.