When comparing Chromium OS vs Haiku, the Slant community recommends Haiku for most people. In the question“What are the best Operating Systems for x86 PCs?” Haiku is ranked 3rd while Chromium OS is ranked 6th.
Ranked in these QuestionsQuestion Ranking
Pro Minimalistic interface
Chromium's system UI uses as little screen space as possible by combining apps and standard web pages into a minimal tab strip: While existing operating systems have web tabs and native applications in two separate strips, Chromium OS combines these, giving access to everything from one strip. The tab is the equivalent of a desktop application's title bar; the frame containing the tabs is a simple mechanism for managing sets of those applications and pages.
Because Chromium OS is designed for users who spend most of their computer time on the Web, it is intended for use in computers with little local storage and fast boot-up times.
Pro Safe and requires little to no maintenant
Since you only have the browser, you cannot install applications (outside Chrome Apps which are decently protected by their limited API and the Chrome Web Store). This means no harmful virus, and also nearly no way you can screw your system. Everything will just work.
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
Possibly harms your privacy.
Con No auto-upgrade (unlike Chrome OS)
Chrome OS auto-upgrade the system, but Chromium OS does not. It's possible that they're thinking about adding that feature from the design doc.
Con No one uses it
It's a very niche OS that no one uses.
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.
It's still in beta and quite unstable. Making it unsuitable for developing applications of any kind.