After entering a value for the Planck constant (https://www.wikidata.org/wiki/Q122894) in terms of its SI unit joule-seconds, the value of 6.6x10^-34 displays in wikidata as 0.0000000000 joule-second (and the uncertainty value also disappears). This isn't very useful. In a similar vein, entering the half-life of Bi-209 (https://www.wikidata.org/wiki/Q18888193) which is 1.9 +- 0.2 x 10^19 years displays in wikidata as 19,000,000,000,000,000,000±2,000,000,000,000,000,000, a little hard to read. I think python's 'g' format defaults are reasonable - for anything below 10^-4 or above 10^6 (or above the length of the number in the default precision) display in scientific notation with an 'e', otherwise display as a regular number. Or quantity display format could perhaps be a user-specified setting as with language.