Preamble:

I've so far only used my UHK for "regular text" usage, i.e. writing text, CLI, browsing and some light wasd + trackball gaming...

I've also activated an option for the left Mod key so that pressed alone it gives me a space character but together with other keys it works as Mod.'

I have the brown switches which I find satisfyingly clicky enough and at the same time easy to press.

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* arrow keys

are very comfortable with holding my left thumb on Mod down and then using the right fingers for jkl and i (I was a bit skeptical at first but even the left thumb on Mod + left pinky on Ctrl to jump words etc. is working well for my fingers).

In some instances I've also used the right thumb on the frame switch ( Mod; which I find not as comfortable to press as the regular Mod/Space keys but works well enough) plus jkli for one-handed operation.
Also, if you need to do a lot of arrow-work at once you can toggle the Mod key held by double-tapping it (same for the mouse and fn keys).

For applications / games where you use the arrow keys and other keys at the same time a lot and where input speed is critical I would look into creating a custom mapping though, if possible. It's very easy to duplicate an existing layout and modify it with the UHK Agent (and also to switch between layouts. Later on a feature to automatically switch layouts depending on foreground application is planned, IIRC).

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* ESC

left-handed Mod+q has become almost second nature after a few "training" sessions with vim :) and if you don't want the burden to hit the q key exactly, Mod+` (key in the upper left corner) works as well in the standard mapping.

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* F1-F12

I barely had the need to use them since I switched to the UHK.

I can imagine that it might be an issue in e.g. certain games where you would want to use F1-F12, 1-12 and Ctrl+1-12 and Ctrl+F1-F12 at the same time and quickly.
For the occasional F2 or F4 in htop it is more of a mental hassle (for me) to figure out if the description is supposed to be left or right of the F* label than pressing Mod+number, especially since the F* are printed on the vertical sides of the caps.

Besides, if there was an extra row of keys above the numbers I wouldn't be able to reach them with my fingers anyway without lifting my hands from the (excellent! wooden) palm rest.

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* thumbs vs. weak fingers

there are extra switches on the frame right "below" the split mod/space key which you can use or reconfigure the same as any other key (if you don't want to have secondary actions like I have set up with the left Mod key). One of the addons (Key Cluster Module) will also bring extra keys into the split.

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Granted, SysRq makes you think a while before you get it right the first time :)

One of the biggest advantages yet for me has been the ability to put my trackball where the numpad used to be and thus have it in an even more comfortable position for my right arm and shoulder. I'm looking forward to the addons though to see if they will obsolete the need for any extra pointing device.

That brings me to my biggest hurdle so far: I've yet to create a numpad-layout to more quickly enter lots of numbers and that will definitely feel just weird on a staggered key layout compared to a regular numpad (after half a decade of very frequent and regular usage). If all attempts to retrain my fingers on that would fail there are still those USB-numpads availble though for the now more occasional number-after-number sessions and it's an expense and inconvenience I am willing to potentially endure for the satisfaction the UHK gives me so far in all other typing and ergonomic aspects.

HTH

PS: all and any typing and other errors are mine and not the UHK's ;)

[EDIT: formatting]