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>- How compatible are they with modern hardware? I'm talking about Windows 10 era desktops and laptops with wifi cards.
Depending on your use-cases both can be a main OS for you and none aswell. Both OS lacks on 3d acceleration front, but OBSD have pretty good wlan drivers. Haiku now uses lan and wifi drivers both from FreeBSD and from OpenBSD, so there should be no big differences in hw support. However Haiku doesn't support some uncommon setup or hw.
Sound support: it is OK with OBSD, but can be tricky with Haiku, in some cases the OpenSound package can help (instal lfrom the Depot), or in some cases a soft/warm reboot from a different OS can help to fix the initialization.
Neither Haiku nor OpenBSD have usable Bluetooth stack, however Haiku at least have the basics already in place, so basic pairing works, but nothing else. Do not expect your bt speaker will work.
OpenBSD have bigger development team but they try to support a much wider architecture, while Haiku practically available only for x86/x86_64 and for riscv platforms. Arm is in progress, but nobody reached desktop yet.
I don't really own modern hw, but i always test Haiku with my company provided computers, so far i had positive experiences.
Haiku supports NVME SSDs by the way.
If you need any other info, let me know.
- What programs are available other than first-party software? Are there developer friendly tools available?
Both for Haiku and for OBSD you can research the available ports via a webbrowser:
https://depot.haiku-os.org/
and
https://www.ports.to/ OR https://openbsd.app/
- What's the state of the GUI? Is it as usable as Windows XP/7? Is it customizable?
On BSD you can install many well known DE, so customization shouldn't be an issue. However on Haiku it is really different. There is only one DE, and while it provides some customization, you won't win a unixpr0n rice-price with Haiku. There are themed window frames and control UIs (window gui elements), but i prefer the default one, so i stick with that. You can however color it as you wish, some made dark themes, but i don't feel the need for dark UI, so i don't care.
For me the Haiku UI is usable, clear and simple, but keep in mind i use BeOS like OS since 2000 or so. A newcomer should probably unlearn many things, and adapt / learn about many BeOS/Haiku specific stuff. The earlier you accept it is a unix like but not unix system (eg. not linux, not bsd, it doesn't provides multiuser capability on the GUI, there is no ACLs and many other things many takes as granted.
If you have any question, feel free to ask.