What are some non-functional features of a chess clock that would help choose the best for ones use? All chess clocks time the match. Some just time, some add delays, some add on, and other have variations for TIMING.

What else might be a factor in deciding to buy a clock that is primarily for personal use at speed chess in a club, cafe, or park,  but also useful should one play in tournaments? For examples, cost,  reliability, and ease of use are some factors. What would be the features that would help guide a purchase?

There might be other subtler ones, so a complete list for consideration is what is asked for.  Comments on why would be useful too.  EG what value does some certification add, if any.

I am thinking there could be easily a dozen or more of these NON-FUNCTIONAL characteristics.
Well I learned way more than I wanted to know about other considerations than mere timing.

This table may be collapsed into a long string of text
someone please edit if that happens 

Cost
Size
Accuracy
Precision
Ease of use
Color
Battery Life
Reliability
Ruggedness
Toggle method
maintenance required
Flag display
Analog/digital
Shape
Aesthetic
Ease of setting
Read out type
Safety against accidental reset
Counts moves
Useful for other than chess  

Timing variations
 - pre/post add
 - pre/post delay
 - fischer
 - bronstein
 - hourglass - seesaw
 - byo-yomi
 - canadian OT
 - guillotine
 - bonus timing
 - different periods timings

custom settings -MODIFY built in
custom settings NEW
Number of custom settings