Hi,
"well, even easiness and usability are personal judgements."
Well, Yes and No

Of course does personal judgement play an important role, but there are certain common design rules that lead to an easy and intuitive layout.
Examples:
Rigol DP832A:
- distance between the connectors is all the same and quite close --> arrange + and - connectors as (channel)groups for easy identification
- Channel On/Off switches left-to-right arrangement (1-2-3) differs from that of the Display (1-3-2) --> no words, just

- Channel On/Off switches situated just on the opposite rim of the casing far off of the associated connectors --> simply

- "All" channel button appears logically as part of the encoder/Keypad block -->

- radially segmented display with almost an overkill of information and hence mostly very small Fonts --> arrange either horizontaly or vertically, reduce the number count of infos and allow for large easy to read Fonts for the most important infos.
- arrangement of infos differ between channels. For ch 1 and 2 it´s vertical, for ch3 its mixed hor/vert -->

- redundant infos on a anyhow crowded display. The center circle repeats the set value of the highlighted channel --> why??

- 36 buttons, 1 knob, 1 switch -->

- 11 (!!! if I counted correctly) different button shapes -->

- and finally -and just for optics- the mains power switch
Of course it is not easy to layout the front for intuitive easy handling when you have to cope with so much functionality.
Still Rigol could have done a much much better job here ... and on almost all of their new devices.
They employ hundreds of engineers in their R&D department .... but why not a single competent industrial designer?
You expected a same layout within the family? Well, not with Rigol darling!

Rigol DP831A
- channel arrangement in the display is 2-1-3 -->

- channel connectors layout is not highvoltage/highvoltage/lowvoltage (DP832A) but just the opposite LV/HV/HV
Siglent SPD3303X
- connectors are arranged in groups -->

- On/Off buttons in proximity to the connectors -->

- channel arrangement in display coincides with connector layout -->

- no display for third channel -->

- Fonts for ´secondary´ infos very small -->

- 15 buttons in only 3 shapes and arranged logically in groups -->

- noy keypad for quick and easy programming -->

- CC/CV LEDS situated right above the channel´s connectors -->

Siglent SPD3303C
- connectors are arranged in groups -->

- On/Off buttons in proximity to the connectors -->

- channel arrangement in display coincides with connector layout -->

- no display for third channel -->

- only most important infos shown -->

- large easy to read and color-coded Fonts -->

- 15 buttons in only 3 shapes and arranged logically in groups -->

- CC/CV LEDS situated right above the channel´s connectors -->

The Siglent layouts are not perfect either, but they certainly did a much better job than Rigol.
Display infos are easier to recognize and to read, with the 3303C beeing the easiest (admittedly no big deal to get that title with the least of infos on display).
Buttons, knobs and connectors are clearly arranged in amore logical way and are more intuitive to handle.
If it were for same specs and functionality I´d rather choose the device with the clearly better layouted front and UI.
regards
Calvin