What do we know that works 100% with Haiku?
It very much depends on how much you're willing to redefine 100%. The easiest barn side to hit is "it makes some sort of use of it" let's call that 25% working. Haiku does pretty well here for PC graphics cards via VESA, for most ATA controllers and drives, and for trivial peripherals like keyboards. It has some coverage for network cards, USB storage, and audio.
But a lot of people expect more, what I'd call 90%, they want the features of their hardware exploited, and on that Haiku does pretty poorly. Almost every component has a lot of nice features that Haiku can't use or doesn't try to. This ends up being a big waste of power and performance.
And if Haiku's supposed to be "better" than Windows for desktop users they're also going to wonder where that last 10% went, and that's all in software. This is the place where reviewers often score Linux distributions short. They bought a cheap device for their PC which says it has "3D stereo sound" and it has a "True 3D" button on their toolbar, so they want to know why Linux doesn't "support" the true 3D feature. The uncomfortable truth is that there is no "feature", it's a cheap audio processing trick in software bundled with the device. To get to that 100% score Haiku's developers need to replicate hundreds of features implemented in the software drivers for these products. It's not fair, but that's the situation.
How about the old BeOS supported hardware. Are they still supported?
There's no good rule of thumb for this. In cases where BeOS supported hardware that was already pretty ancient (ISA network cards? serial port modems?) there's generally been no development effort on Haiku to support them because no-one much cares.