Software is as critical as hardware: The UHK supports holding a key down to select a different keymap layer, then using the other hand to tap a key in that layer. While few people learn many-key chording, "one finger each hand" is every bit as easy as ordinary typing, with practice. This supports an n^2 expansion in effective keyboard reach. I'm surprised this isn't the norm for selecting Chinese/Japanese characters.
The question is: How well? I'm not entirely happy with my QMK firmware implementation. Tap-hold is the antithesis of n-key rollover; one needs to learn to properly nest key-down, key-up events. It's easy to switch from a clarinet (cleanly cover holes) to a saxophone (slap pads), but hard switching back. And we've all learned to type like we're on a saxophone, relying on n-key rollover to cover many sins.
I now use QMK firmware with Hasu controllers for multiple Leopold FC660C keyboards with Topre Silent 45g "rubber dome" key switches. After trying dozens of mechanical key switches, I found that nothing feels like a Topre switch.
And here's the kicker: The physical form factor doesn't matter so much if your fingers rarely go far from home row, which is what tap/hold facilitates.