My highly customizable split mechanical keyboard. Not naming the product because it's my fault that I regret it. It's very high quality and does exactly what it claims to do, and I'm sure many users are happy with it. It cost nearly $400.

It's a ~60% keyboard, which means it's missing a lot of keys. Notably the F keys and the navigation keys (pgup, pgdn, ins, del, etc). There are ways to remap the blank keys to what you really need, and of course there are all the key sequences and chords you can use. But you have to memorize this stuff, because many of the keys are unlabeled by necessity.

I use it on my PC, Mac (work), and Linux (steamdeck). It's a lot of cognitive burden remembering shortcuts and which modifier keys do what on which OS, with the added headache of having to remember which unlabeled or remapped keys are which modifier keys. I love the feel of it and for normal typing it's great, but anytime I stray from the basic keys (A-Z, numbers, etc) it becomes difficult. I end up doing embarrassing things like using right click on my mouse to do things like copy/paste.

If I was on exactly one OS all day long, I think I could make it work. But juggling 3 is annoying.

If I could redo it all over again I'd realize I didn't need a split ergo keyboard, and would have gotten something more traditional.