Usability
Interface 
User-friendliness/complexity of the tool´s interface
Straightforward.
Time efficiency Time needed for mastering the „how-to“ of the tool and a production of a task
Medium; users have to get familiarised with the instructions and working with Google Sheets, but then it becomes fairly easy to use.
Interface language(s) The number of languages in which the tool is currently available/adaptable to
English.
Respect for privacy Does the tool require registration? Who can see my data? Can I remain anonymous?
High (?). Timeline JS does not contain a privacy policy and is thus difficult to assess. The website does not seem to collect cookies and the whole project is open source, not-for-profit, and designed for educational purposes only. On the other hand, timelines are public, but it is unlikely that someone without the timeline link finds it through search engines. See more here.
Saving work for future use Possiblity to save your or others´ work for reuse/reproduction
Yes, users can access and edit their timeline as long as they have the related Google Spreadsheet.
Tailorability (If so, how?) Possibility of the tool to be used for a range of different activities (e.g. to what extent you can organize a project assignment with a central activity based on this tool)
To some extent; users can employ Timeline JS to produce timelines about virtually anything, although its features are rather restricted.